post https://api-sandbox.bill.com/api/v2/RecordARPayment.json
Record a payment received outside BILL. You can apply the recorded payment to specific invoices or let the payment stay unapplied. An unapplied payment can occur because of overpayment, double payment, or a customer payment not currently linked to an invoice.
In the response, a ReceivedPay object is generated for the offline received payment.