What is the ruling on purchasing an apartment, part of whose price was paid as a maintenance deposit in a usurious bank, and what is the obligatory action for the buyer, and is he required to inform another buyer when selling the apartment later?

The presence of a maintenance fund in a usurious bank account—paid by the previous owner and obligatory for you to pay—does not affect the validity of the apartment purchase contract, nor does it incur sin upon you. It is advisable to counsel the person responsible for the funds to deposit them in an Islamic bank. If he does not comply, the burden of sin falls upon him, and you are not obligated to sell the apartment or vacate it, for "no harm will come to you from those who stray if you are guided."