Do you have the credit with the same seller, from a previous purchase?
Ask the seller.
Do you have a credit balance in your Paypal account?
If you have less than needed, add more money to the PP account, and make your purchase. If you don't, PP will use your credit card or your bank account and not touch the PP balance.
If you have enough or more than enough, just pay as usual, but make sure you tick off 'pay from PP balance'.
