Is buying an apartment in installments from a relative, with the condition of paying a sum of money as a down payment periodically until the installments are settled, considered usury? And upon whom does the burden of sin fall? And what should be done?

Installment sales are permissible if the two parties agree upon a specific deferred price. However, it is not permissible to increase the price in exchange for a longer payment period or when the buyer delays payment, because that is usury (riba). The matter of a debtor who procrastinates should be referred to the judiciary, and the one in hardship must be given respite. If the contract includes something prohibited, then the sin falls upon both parties if they are aware of the prohibition, and repentance is obligatory, as is returning the usurious increase.