Allow custom payment amounts for Open Item (Spruce)

Summary: Customers on Open Item accounts want to enter a flat payment amount (partial or lump sum) instead of selecting invoices.
Problem: EvoX requires invoice selection, which doesn’t match how payments are often made in Spruce. Spruce allows for partial payments of invoices where the partial amount is applied to the invoice. EvoX also has a total and AR paid column which would allow the remaining balance to be shown
Request: Add a “custom amount” option so users can pay any amount and apply it across invoices.