This can be accomplished by using a unique Accounts Payable account in place of the Cash GL account in the Cash field of the Grant. When the credit card payment is entered, you would use the new AP account in the Expense account field of the AP voucher.
First you need to create an account in the General Ledger called Credit Card Payable (or something similar). This will need to be added to the fund making the grant, or whatever funds you need to via Mass-Add General ledger Accounts.
Before you enter the application go to the grantee tab - make sure the credit card company is a payee on the contact tab. On the grantee tab - under the payee radial button - select the credit card company.
Before you post the application in the grant module - go to the fund module and to the fund paying this particular grant. Click on the GL Override tab and on the line for "Grant Check Credit" - change it from the Grant Payable account to the Credit Card Liability account.
Post that application and then remember to go back and take that off of the GL Override tab for that fund. (You just want to edit the system for that application only).