> ## Documentation Index > Fetch the complete documentation index at: https://docs.arcuserp.com/llms.txt > Use this file to discover all available pages before exploring further. # Apply a single payment to multiple invoices (atomic create) > Atomic-create endpoint for multi-invoice AR cash payment application. Accepts a complete payment + applications payload in ONE transactional call. Creates one order_payments row (order_id = NULL) and N order_payment_applications rows. Validates sum(applications[].amount_applied) <= amount within $0.005. Each invoice's balance_due must cover its application. Posts one GL journal entry for the applied portion (DR Cash-equivalent / CR AR). Broadcasts payment.applied_multi_invoice + per-invoice order.payment_status_changed events. Industry standard: NetSuite Customer Payment + Auto Apply, QuickBooks Payment Line[] + unapplied credit, Acumatica AR Document Applications + Auto-Application. Idempotent via Idempotency-Key header. Auto-apply: pass auto_apply=true (and omit applications) to have the server allocate the payment oldest-first across the account open invoices (capped per invoice balance_due). The response returns the computed applications[]. Unapplied cash: if the amount exceeds what the open invoices can absorb, pass route_unapplied='credit' to hold the surplus as a credit-on-account (a balanced DR clearing / CR Customer Credits journal entry posts in the same transaction and the account credit_balance is incremented). Without route_unapplied, a surplus is REFUSED (no silent cap). ## OpenAPI ````yaml /openapi.yaml post /payments/multi-invoice-apply openapi: 3.1.0 info: title: Arcus ERP Public API version: 1.0.0 description: > Arcus ERP public REST API. Designed for external integrations and data migration. **Authentication.** Bearer token (API key) via the `Authorization` header. Format: `Authorization: Bearer ark_live_ent__` (or `ark_test_*` for sandbox). API keys are issued per-entity in **Settings > Developers > API Keys**. **Entity scoping.** The entity is encoded in the API key prefix; routes are flat (e.g. `/v1/accounts`, `/v1/orders`, `/v1/products`). A small set of platform endpoints (migration, reconciliation, events, webhook endpoints, API keys) use the `/v1/entities/{entity_id}/...` form -- those are noted in their tags. **Key capabilities.** - Related-resource hydration via `?expand[]=` (see `x-arcus-expand` on each resource). - Cursor-based pagination (`starting_after` / `ending_before` / `limit`). - Idempotency via the `Idempotency-Key` header. - Webhook events for asynchronous notification. - Conditional requests / ETag for cache validation. servers: - url: https://api.arcuserp.com/v1 description: Arcus ERP API (accepts both live `ark_live_*` and test `ark_test_*` keys) - url: https://dev-api.arcuserp.com/v1 description: >- Dev sandbox API (test-only data, accepts `ark_test_*` keys against dev RDS) security: [] paths: /payments/multi-invoice-apply: post: tags: - Payments summary: Apply a single payment to multiple invoices (atomic create) description: > Atomic-create endpoint for multi-invoice AR cash payment application. Accepts a complete payment + applications payload in ONE transactional call. Creates one order_payments row (order_id = NULL) and N order_payment_applications rows. Validates sum(applications[].amount_applied) <= amount within $0.005. Each invoice's balance_due must cover its application. Posts one GL journal entry for the applied portion (DR Cash-equivalent / CR AR). Broadcasts payment.applied_multi_invoice + per-invoice order.payment_status_changed events. Industry standard: NetSuite Customer Payment + Auto Apply, QuickBooks Payment Line[] + unapplied credit, Acumatica AR Document Applications + Auto-Application. Idempotent via Idempotency-Key header. Auto-apply: pass auto_apply=true (and omit applications) to have the server allocate the payment oldest-first across the account open invoices (capped per invoice balance_due). The response returns the computed applications[]. Unapplied cash: if the amount exceeds what the open invoices can absorb, pass route_unapplied='credit' to hold the surplus as a credit-on-account (a balanced DR clearing / CR Customer Credits journal entry posts in the same transaction and the account credit_balance is incremented). Without route_unapplied, a surplus is REFUSED (no silent cap). operationId: applyPaymentToInvoices parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: required: true content: application/json: schema: type: object required: - account_id - payment_method - amount properties: account_id: type: string format: uuid description: The customer account receiving this payment. payment_method: type: string enum: - check - ach - wire - external - cash - credit description: Tender method for this payment. amount: type: number description: >- Total payment amount in dollars (cash actually received). The sum of applications[].amount_applied must be <= amount; any surplus requires route_unapplied='credit'. example: 5000 auto_apply: type: boolean description: >- When true (and applications omitted), the server allocates the payment oldest-first across the account open invoices. The response returns the computed applications[]. example: true route_unapplied: type: string enum: - credit description: >- When 'credit', any surplus (amount beyond what open invoices absorb) is held as a credit-on-account via a balanced journal entry in the same transaction. Without it, a surplus is refused (no silent cap). applications: type: array minItems: 1 maxItems: 50 description: >- Per-invoice application lines (manual override). Omit when auto_apply=true. Sum of amount_applied must be <= amount; any remainder requires route_unapplied='credit'. items: type: object required: - order_id - amount_applied properties: order_id: type: string format: uuid description: >- Invoice (order) to apply this portion of the payment to. amount_applied: type: number description: >- Dollar amount to apply to this invoice. Must be > 0 and <= invoice balance_due. example: 2500 check_number: type: string description: Check number (for check payments). check_date: type: string format: date description: Check date (YYYY-MM-DD). external_reference: type: string description: External reference (ACH trace number, wire memo, etc.). notes: type: string description: Internal notes for this payment. payment_date: type: string format: date description: Payment date for GL posting (YYYY-MM-DD; defaults to today). responses: '200': description: Payment applied successfully content: application/json: schema: type: object properties: data: type: object properties: id: type: string format: uuid entity_id: type: string format: uuid amount: type: number payment_method: type: string status: type: string journal_entry_id: type: string format: uuid applied_count: type: integer applications: type: array items: type: object properties: id: type: string format: uuid order_id: type: string format: uuid amount_applied: type: number idempotent_replay: type: boolean '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '422': $ref: '#/components/responses/Error' security: - ApiKeyAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header required: false description: | Client-generated unique key for idempotent POST/PATCH/DELETE operations. Alias for the Idempotency parameter. Max 255 chars. On retry with the same key, the original response is returned without re-executing the operation. Keys expire after 24 hours. schema: type: string maxLength: 255 responses: Error: description: Error response (400/401/403/404/409/422/429/500) content: application/json: schema: $ref: '#/components/schemas/ErrorEnvelope' schemas: ErrorEnvelope: type: object description: | Canonical error response envelope. All API errors use this shape. required: - error - code properties: error: type: string description: Machine-readable error key example: not_found code: type: string description: Machine-readable error code (often same as error) example: not_found type: type: string enum: - validation_error - permission_error - not_found - conflict - rate_limit - internal - expand_error - not_implemented example: not_found hint: type: string description: Human-readable one-sentence explanation (English) example: >- The requested order does not exist or does not belong to this entity. param: type: string description: The parameter that caused the error, if applicable example: expand[0] required: type: string description: The scope required (only on insufficient_scope errors) example: accounts:read request_id: type: string description: >- Unique request ID for support tracing (maps to CloudWatch log stream) example: req_abc123 securitySchemes: ApiKeyAuth: type: http scheme: bearer description: | API key issued per entity via Settings > Developers > API Keys. Each key carries scopes (e.g. orders:read, products:write). Bearer token format: Authorization: Bearer ark_live_ent__ Test keys use ark_test_ent__. Both are issued per entity via Settings > Developers > API Keys. ````