Receivable Invoice generated from PPM Contract should have manual invoice number

We are generating all the receivable invoices from PPM Project and Contract and then transferring them to the receivables module. On the contract, we have Amount based bill plan and a billing event will be created on the contract to generate sales invoices.

We are interfacing billing amounts from a boundary application to create a billing event. And our requirement is to retain the invoice number from that application. If we use manual invoice numbering on "Customer Contract Management Business Function" configuration then at the time of generating the invoices or at the time of release we have to provide the Invoice number.