Is it permissible or forbidden to take the penalty clause stipulated in the contract between the seller and the buyer if one of the parties breaches the terms of the contract (such as the seller backing out a week after receiving the down payment for the sale)?

If two people agree to purchase something in the future, there is no harm in that. It is permissible to document this promise with a penalty clause to be paid by the party who reneges, as there is no legal impediment. However, if the sale contract has been concluded, the seller has received the price, and they have parted ways, the seller does not have the right to back out and is compelled to deliver the sold item, unless the buyer grants rescission. Likewise, the buyer does not have the right to back out after the contract is finalized.