If your MIL and DH are equal owners of the property, he has the right to an equal say. You might consider the possibility of splitting the property into two parcels, with ownership of one given to your DH and the other to you MIL; then you can price your property for whatever it will actually sell for. I am sure there are legalities for subdiiding the property, so you would need to check into that, and you also need to find someone who can make an impartial and equal recommendation on where the split should be.