Should the price of the land be calculated at its current value or at its value when part of its price was taken in the past, after a year and a half has passed since the incomplete initial agreement?

It is impermissible to sell land for an unknown price or for an unknown term, due to the presence of gharar (excessive uncertainty) which invalidates the sale. Therefore, the price and the term must be known to both the seller and the buyer. If the sale was concluded in this manner, it must be annulled and the situation restored to its original state. Then, they should re-conclude the sale with a known and specified price.