I would suggest that you have the confidence to accept the money.If you don't want to keep it, you can donate it to charity.However, the problem is solved if you accept the money.Your neighbours are obviously not "users"and that's possibly why you like them.So don't make them feel uncomfortable by not letting them pay.They won't give you the opportunity to assist them again and will go somewhere else next time.No one would think you are ripping them off if you ask/accept a fair price.You will enjoy the project once the finances are settled.