+// the user has chosen an invoice. the previously chosen invoice is still
+// in curr_invoice
+// - if there is a value there, put it back on the invoices_for_row list for
+// this customer.
+// - then _remove_ the newly chosen invoice from that list.
+// - find the "owed" element for this application row and set its value to the
+// amount owed on that invoice.
+// - find the "amount" element for this application row and set its value to
+// either "owed" or the remaining payment amount, whichever is less.
+// - call change_app_amount() on that element.