Is there a sin upon the intermediary in a contract for the purchase of an apartment, which later turned out to be with usurious installments, despite the buyer's contentment with it?

If the brother bought an apartment from the bank at a known deferred price higher than its cash price, this is not considered usury (riba) as long as the price does not increase with the delay in payment. However, if the apartment was purchased with a usurious loan from the bank, then it is forbidden usury regardless of the consent of the contracting parties, because their consent does not make the forbidden permissible, and usury is one of the major sins.

As for the questioner who witnessed the contract: If the contract was of the first type (non-usurious), then there is no sin upon him. But if the contract was usurious, then it is not permissible to witness it or assist in it, due to the saying of the Prophet, peace and blessings be upon him: "Allah has cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses," and he said: "They are all equal."

If the questioner was ignorant of the ruling or the situation, then we ask Allah to excuse him for his ignorance. But if he witnessed it knowing the unlawfulness of the contract, then he has committed a grave sin and must hasten to repent.