If you scheduled the surgery for 2000 grafts and they took out 2500, they should install the overage free of charge. It was the doctors mistake that he took too much tissue with grafts.
For the doctor to throw the extra tissue with grafts in the medical waste bin is a sin and unethical. It's also unethical for the doctor to pressure to PAY for the extra grafts as you are in a difficult position to make any decision while your head is full of holes and a strip cut out of your head all sutured up.
Installing the extra grafts is no big deal anyway because the technicians do it. This scam is nothing more than a bait and switch sales method.
The doctor should be reported to the state medical board.