> ## 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 vendor prepayment to a bill > Applies some or all of an unapplied vendor prepayment balance to a specific vendor bill. Decrements `vendor_bills.balance_due` by the applied amount (Rule 23 SSOT). **GL entry posted:** ``` DR Accounts Payable (ap_account) CR Vendor Prepayments Asset (1440) ``` **Validation:** - Prepayment must be in `active` or `partially_applied` status. - `amount` must not exceed `prepayment.balance` (returns 422 `amount_exceeds_prepayment_balance`). - `amount` must not exceed `bill.balance_due` (returns 422 `amount_exceeds_bill_balance_due`). - Bill vendor must match prepayment vendor (returns 403 `vendor_mismatch`). - If `amount` is omitted, applies `min(prepayment.balance, bill.balance_due)` automatically. Requires `ap:write` scope. ## OpenAPI ````yaml /openapi.yaml post /vendor-prepayments/{id}/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: /vendor-prepayments/{id}/apply: parameters: - name: id in: path required: true schema: type: string format: uuid description: Vendor prepayment UUID. - name: Idempotency-Key in: header required: false schema: type: string format: uuid description: Optional idempotency key. post: tags: - Purchasing summary: Apply a vendor prepayment to a bill description: > Applies some or all of an unapplied vendor prepayment balance to a specific vendor bill. Decrements `vendor_bills.balance_due` by the applied amount (Rule 23 SSOT). **GL entry posted:** ``` DR Accounts Payable (ap_account) CR Vendor Prepayments Asset (1440) ``` **Validation:** - Prepayment must be in `active` or `partially_applied` status. - `amount` must not exceed `prepayment.balance` (returns 422 `amount_exceeds_prepayment_balance`). - `amount` must not exceed `bill.balance_due` (returns 422 `amount_exceeds_bill_balance_due`). - Bill vendor must match prepayment vendor (returns 403 `vendor_mismatch`). - If `amount` is omitted, applies `min(prepayment.balance, bill.balance_due)` automatically. Requires `ap:write` scope. operationId: applyVendorPrepaymentV1 requestBody: required: true content: application/json: schema: type: object required: - bill_id properties: bill_id: type: string format: uuid description: UUID of the vendor bill to apply the prepayment against. amount: type: number format: double nullable: true description: >- Amount to apply. If omitted, applies min(prepayment.balance, bill.balance_due). example: bill_id: b1234567-0000-0000-0000-000000000001 amount: 2500 responses: '200': description: Prepayment applied to bill content: application/json: schema: type: object properties: data: type: object properties: application_id: type: string format: uuid prepayment_id: type: string format: uuid prepayment_number: type: string prepayment_balance: type: number description: Remaining balance after apply. bill_balance_due: type: number description: Bill balance_due after apply. amount_applied: type: number journal_entry_id: type: string format: uuid '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '404': $ref: '#/components/responses/Error' '422': $ref: '#/components/responses/Error' '500': $ref: '#/components/responses/Error' security: - ApiKeyAuth: [] components: 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. ````