You can add full or partial payments against an invoice. First, find the invoice to be paid, then follow these instructions:
In the Paid Total column select the green '+' symbol on the item to be paid
Within the Add Payment dialogue box enter the amount paid with payment type
Untick Fully Paid if specifying an amount different to the total remaining outstanding balance
Click Add Payment
Please Note
If a payment has been made that is not the full amount, the system will instantly display the remaining balance in the O/S Balance column