ZoHo One > ZoHo Books (with ZoHo Inventory and ZoHo CRM linked and activated)

I have a contact who is both a customer and a vendor, they would like to apply a Credit Note to reduce the open Vendor Credit balance they have. Here is the scenario:

Contact has a $25 credit note and they are wanting to apply that to reduce their $500 Vendor Credit, leaving an open Vendor Credit balance of $475 on the account.

What is the best way to do this in ZoHo Books (via ZoHo One)? Ideally I would like to make sure I have some form of 'paper trail' that clearly shows the $25 Credit Note was applied toward the $500 Vendor Credit.

I hope that makes sense and thank you for your help!