Are we entitled to reclaim the amount paid, taking into account the current value of the land, or only the amount originally paid, knowing that the heirs stated that the father never took possession of the land in the first place?

If the seller sold land he did not own, the contract is void, and the land remains the property of the association. The seller's heirs must only return the amount paid by the buyer, and they are not obliged to pay an increase on the grounds of the land's price appreciation or the investment of the amount, because the land does not belong to them or to the buyer, and the amount was under their guarantee with the buyer's consent, and "profit is by virtue of liability" (al-kharāj bi al-ḍamān).