Records a payment against an expense's outstanding amount. amount defaults to the full outstanding amount when omitted; a payment exceeding the outstanding amount, or against an already fully paid expense, is rejected with 400. payment_type must be a configured payment-method key (e.g. cash, card, cheque, bank-transfer).

A method configured with refRequired needs a non-blank reference, and a bank-transfer method with transferReceiptRequired needs a receipt_file_id; a payment missing either is rejected with 422 (reference / receipt_file_id).