From the legal perspective, they HAVE TO move as per the deadline stated in the Sales and Purchase Agreement. If not, they would be considered as not obliding to the terms and the transaction can be cancelled and they have to make compensation.
If they are allowed to stay after the transaction is completed, your daddy would lose the upper land. If he wants them to move out later, the procedure is more complicated. The key point is they have recieved the sales proceeds $$$.
Your daddy is so clever to think of this solution. Hence, don't think your dad would give in.