I'd be very worried that he gets the card and claims there is no money on the card. Not only would he get his $112.50 back, you'd be out the original $50.
I really can't see how you can PROVE there was still money on it when it left your hands. You could easily press the buy button 2 seconds before it is in the mail, or reaches it's destination.
Some sites do not have up to the minute tracking of how much money is on the card. If that happens, you are screwed. From what I've read, eBay/PayPal usually side with the Buyer.
I'd rather take the Neg feedback hit than lose the $50. You can always buy $50 of golf popular products off the website and eBay the actual items. That seems a safer bet than this Buyer with 8 feedbacks of all GCs, especially if the others are at or above face value of the GCs. He could have easily gone to the actual websites and bought them out right for face value.