Is it permissible to stipulate a financial penalty on the buyer if he delays in paying the price of the house after the agreed-upon period?

The condition of returning an increment for delayed debt payment is pure usury (riba) and impermissible. The debtor is not obligated to pay it due to their delay. However, if the debtor repays the debt and, of their own accord, gives an increment, this falls under excellent repayment, which the Prophet (peace be upon him) encouraged. The legitimate way to reclaim one's right is to refer the matter to a judge, who will compel the debtor to pay if they are solvent. If the debtor is in straitened circumstances, they must be granted respite until Allah eases their situation, as Allah Almighty says: "And if there is any debtor in difficulty, then grant him time till it is easy for him to repay; and if you remit it by way of charity, that is better for you if you only knew." Taking an increment on a debt for delay is forbidden by the consensus of scholars.