To apply a payment to an invoice in ArchiOffice/EngineerOffice:
1. On the Billing > Outstanding screen, click the Payment icon to open the Transaction Details screen.
2. Enter Date of the payment.
3. Select Payment as the transaction type.
4. Enter any payment Notes you want to record.
5. Now enter the total amount of payment in the Amount field. Also, specify the amount being applied as payment in the 'Amount to Apply' field.
6. Click the Apply link on the grid to apply the payment to the selected invoice.
7. Click Done to finalize the transaction.