Hi @roundcubefour and welcome!
This is a bit off topic since this thread is about Pull Payments, not Invoices, but anyways.
We are not concerned about fixed source amount payments in invoices since it is assumed that the merchant won’t mark an invoice as complete if they do not receive the amount they requested and hence they won’t fulfill the contract. Consequently, we focus on fixed destination amount payments, which are handled at the transport layer (STREAM).