Receipt of an Overly Generous Gift

My wife and I helped a friend and neighbor load and move some items on their moving day for about 4 hours. We were happy to help as they were in a pinch and had to be out of the house that afternoon. This friend dropped off a thank you card with a $100 gift card. While we appreciate the card and giftcard we feel it is way too much and are not comfortable accepting such a gift. We helped out only as a favor for a long time friend. Any advice would be appreciated.