If you'd like to add credit to your customer's account balance, you can use your reseller account balance, but please note, that for Channel Partners, it is using the available commission. For Whitelabel Partners, it is using your reseller balance.
On your reseller portal, locate the "Customer" (ref 2.) in the “Customers” (ref 1.) tab and select it.
Click on the “Account Balance” (ref 3.) button on the top, and then click "Add credits" (ref 4.).
A window will pop up, fill in the "Description" (ref 1.), and "Amount" (ref 2.), and then click "Save" (ref 3.).