Is it considered usury to agree upon an increase in the land's price for my uncle instead of the stipulated penalty clause, or is it better to فسخ العقد (rescind the contract) and take the penalty clause, or is the latter impermissible?

According to some jurists, a down payment (ʿurbūn) is permissible. This is an amount paid upon concluding a contract, and if the buyer does not complete the transaction, the down payment belongs to the seller. It is permissible for the seller and buyer to agree to annul the contract if the buyer does not pay the price within a specified period. However, it is not permissible to increase the price in exchange for delayed payment, as this constitutes usury (riba). If annulment is not stipulated in case of non-payment, some jurists have allowed annulment when the buyer procrastinates. In this case, the amount paid by the buyer must be returned, and it is not permissible to take the down payment, unless the annulment and taking of the down payment were conditioned upon non-payment within a specific time.