> ## Documentation Index > Fetch the complete documentation index at: https://docs.fynn.eu/llms.txt > Use this file to discover all available pages before exploring further. # Extend a payment plan > Replace every not-yet-paid installment with a new schedule. Already paid installments stay untouched. Send EITHER `installments` OR `split`, exactly like create but without `invoiceId`. The new schedule must sum to the current open amount of the invoice in cents (in split mode the server computes it). A broken plan returns to `active`, and `brokenAt` and `brokenReason` are cleared. Use this instead of creating a new plan when a customer misses an installment or needs more time. ## OpenAPI ````yaml assets/openapi-v2-payment-plans.json post /api/payment-plans/{id}/extend openapi: 3.1.0 info: title: Fynn V2 Payment Plans API version: 2026-07 description: >- REST API for Fynn payment plans (Zahlungsplan). Split an open invoice into interest-free installments while dunning stays paused, let Fynn collect direct-debit installments automatically, and react to the plan lifecycle over webhooks. All amounts are integers in the smallest currency unit (cents). Authenticate with an Organisation API token. servers: - url: https://coreapi.io description: Production - url: https://preview.coreapi.io description: Sandbox security: - ApiToken: [] tags: - name: Payment Plans description: Create, read, cancel and extend the installment plan of an open invoice. paths: /api/payment-plans/{id}/extend: post: tags: - Payment Plans summary: Extend a payment plan description: >- Replace every not-yet-paid installment with a new schedule. Already paid installments stay untouched. Send EITHER `installments` OR `split`, exactly like create but without `invoiceId`. The new schedule must sum to the current open amount of the invoice in cents (in split mode the server computes it). A broken plan returns to `active`, and `brokenAt` and `brokenReason` are cleared. Use this instead of creating a new plan when a customer misses an installment or needs more time. operationId: extendPaymentPlan parameters: - $ref: '#/components/parameters/PaymentPlanId' requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ExtendPaymentPlanInput' responses: '200': description: The extended payment plan. content: application/json: schema: $ref: '#/components/schemas/PaymentPlan' '401': $ref: '#/components/responses/Unauthorized' '403': $ref: '#/components/responses/Forbidden' '404': $ref: '#/components/responses/NotFound' '422': $ref: '#/components/responses/UnprocessableEntity' components: parameters: PaymentPlanId: name: id in: path required: true schema: type: string format: uuid example: 10000000-0000-0000-00e0-000000000001 description: The payment plan UUID. schemas: ExtendPaymentPlanInput: type: object description: >- Like the create body without invoiceId. Send EITHER installments OR split. properties: installments: type: array description: >- Manual mode. Replaces every not-yet-paid installment. Must sum to the current open amount in cents. items: $ref: '#/components/schemas/InstallmentInput' split: $ref: '#/components/schemas/SplitInput' PaymentPlan: type: object properties: id: type: string format: uuid example: 10000000-0000-0000-00e0-000000000001 invoiceId: type: string format: uuid example: 10000000-0000-0000-0003-000000000001 status: $ref: '#/components/schemas/PlanStatus' totalAmount: type: integer description: >- Sum of all installments in cents. Equals the open amount at creation. example: 1190 currencyCode: type: string example: EUR paymentMethodId: type: string format: uuid nullable: true example: 10000000-0000-0000-004f-000000000001 createdAt: type: string format: date-time example: '2026-07-05T16:20:00+00:00' completedAt: type: string format: date-time nullable: true description: Timestamp the plan completed (all installments paid), or null. example: null brokenAt: type: string format: date-time nullable: true description: Timestamp the plan broke, or null. example: null canceledAt: type: string format: date-time nullable: true description: Timestamp the plan was canceled, or null. example: null brokenReason: type: string nullable: true description: Reason the plan broke, or null while active. example: null installments: type: array items: $ref: '#/components/schemas/Installment' required: - id - invoiceId - status - totalAmount - currencyCode - paymentMethodId - createdAt - completedAt - brokenAt - canceledAt - brokenReason - installments InstallmentInput: type: object description: One installment in manual mode. properties: dueAt: type: string format: date description: >- Due date of the installment (Y-m-d). Must lie in the future and be strictly after the previous installment. example: '2026-08-05' amount: type: integer description: >- Installment amount in the smallest currency unit (cents). Must be positive. example: 595 required: - dueAt - amount SplitInput: type: object description: Even split of the open amount into a number of installments. properties: count: type: integer minimum: 2 maximum: 36 description: Number of installments to create. example: 3 interval: type: string description: >- Distance between due dates as {n}{D|W|M|Y}, for example 1M for one month. Defaults to 1M. pattern: ^[0-9]+[DWMY]$ default: 1M example: 1M firstDueAt: type: string format: date description: Due date of the first installment (Y-m-d). Must lie in the future. example: '2026-08-05' required: - count - firstDueAt PlanStatus: type: string description: Lifecycle status of a payment plan. enum: - active - completed - broken - canceled Installment: type: object properties: id: type: string format: uuid position: type: integer description: 1-based position within the plan. example: 1 dueAt: type: string format: date description: Due date (Y-m-d). example: '2026-08-05' amount: type: integer description: Installment amount in cents. example: 595 status: $ref: '#/components/schemas/InstallmentStatus' paidAt: type: string format: date-time nullable: true description: Timestamp the installment was fully covered (ISO 8601), or null. example: null required: - id - position - dueAt - amount - status - paidAt ProblemError: type: object description: >- Problem detail error body (application/problem+json). Additional members may be present depending on the error. properties: status: type: integer example: 422 type: type: string description: Stable machine readable error key. example: PAYMENT_PLAN__SUM_MISMATCH title: type: string example: An error occurred detail: type: string example: The installments must sum to the open amount of the invoice. required: - status - type - title - detail ValidationError: type: object description: >- Validation error body (application/problem+json) returned when input fails field level validation. properties: status: type: integer example: 422 type: type: string example: https://tools.ietf.org/html/rfc2616#section-10 title: type: string example: Validation Failed violations: type: array items: type: object properties: propertyPath: type: string example: installments[0].amount message: type: string example: This value should be positive. code: type: string nullable: true required: - status - type - title - violations InstallmentStatus: type: string description: Status of a single installment. enum: - open - collecting - paid - failed responses: Unauthorized: description: Authentication is missing or the token is invalid. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' Forbidden: description: The token is valid but lacks the required permission. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' NotFound: description: The requested resource does not exist for this Organisation. content: application/problem+json: schema: $ref: '#/components/schemas/ProblemError' UnprocessableEntity: description: The request failed validation or a business rule was violated. content: application/problem+json: schema: oneOf: - $ref: '#/components/schemas/ValidationError' - $ref: '#/components/schemas/ProblemError' securitySchemes: ApiToken: type: http scheme: bearer description: >- Organisation API token. Send it as Authorization: Bearer api_... The token is scoped to a single Organisation. Reading a plan needs the invoice read permission, creating, canceling and extending need the invoice write permission. ````