To apply a credit or prepayment to an existing invoice follow the below instructions.
From the BusinessMind home page click on the Sales Link
From the Invoices section search for the credit note or prepayment you want to apply.
Click on the credit or prepayment to display the detail view.
Click on the gear icon and select Allocate Credit.
This will bring up a list of invoices, fill in the amount to credit field on the invoices you wish to to apply to.
Click the Allocate Credit button to complete the process.