Yes, you do.1) A mistake does not let you keep money (or get the benefit of a payment made on your behalf) if you are not otherwise entitled to it; they could potentially sue you for the money and if they did and can show it was a mistake, will win.2) As you note, a realtor can't contribute on a new home. The contribution could cause problems with your loan if the bank discovers it and wants to make an issue of it (assuming you have a loan) since it is not allowed.