Mike Larry has a random real estate question

My family owns a condo. For the purposes of this question, let's assume the place is worth $750k. The family that lives next door wants to buy our condo in order to knock down the wall and expand their own into a larger space. They said they'd be willing to put the money towards the purchase of another condo and also cover the moving expenses into the new place.



We don't really know this family, so we certainly owe them nothing. In this situation, is this favor worth something, and if so, how much? Would it be fair to present new places worth $800k? $850k?