You can manage the Customer Credit or House Account by opening the 'Customer Credit' window, you can list the customer and check his 'owe history' and 'payback history'. To use the 'pay back' function, select the customer that under the 'customer list', then click the 'pay back' button.
The customer list will be presented in the left column, when a customer selected, the account history and payment history will be displayed in the middle and right columns.
Click the payment button at the bottom of the window will process a payment for a selected customer account.