Is what the questioner did, borrowing a sum of money from his superior at work to pay for a house, and repaying the amount in interest-free installments, while keeping his superior’s savings book until the end of the repayment, considered unlawful, and is he thereby aiding in usury?

You must advise the official and warn him against consuming Riba if he deposits his money in a Riba-based bank and takes interest. It is not permissible for you to deposit the installments into his Riba-based account, based on the words of Allah Almighty: "And do not cooperate in sin and aggression," and based on the words of the Prophet, peace and blessings be upon him: "The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes Riba, the one who gives it, its scribe, and its two witnesses, and he said: 'They are equal.'" You must return the installments to him instead of depositing them into that account.