You are creating new supplier invoice via web service ManageSupplierInvoiceIn, but you don't know how to assign existing down payment requests to the invoice in the payload.
SAP Business ByDesign, All Versions
In current release, it is not supported to assign existing down payment requests to supplier invoice via web service.
When creating new supplier invoice via webservice, existing down payment request will be automatically proposed in the supplier invoice. You can confirm the details under:
Supplier Invoice -> General tab -> Payment Information -> Down Payments.
After the supplier invoice saved in system, you can manually adjust the assigned documents from UI if required.
ManageSupplierInvoiceIn, Supplier Invoice, Down Payment Request, Web Service, API , KBA , SRD-SRM-SI , Supplier Invoicing , Problem