Partial Credit

A partial credit cannot be issued through WSM. If you would like to issue a partial credit, this can be accomplished by logging in to your payment gateway account.

Paypal

https://www.paypal.com/us/webapps/helpcenter/helphub/article/?solutionId=FAQ2560

Authorize.net

http://www.authorize.net/support/CP/helpfiles/Search/Transaction_Detail/Refund_a_Transaction.htm