Is the building watchman's taking a 10% increase in exchange for purchasing consumables and providing laundry and car services from his own money, in addition to a separate monthly salary for other services, considered usury?

It is not permissible to combine a loan with a commutative contract, such as a sale or a lease, because the Prophet, peace and blessings be upon him, said: "It is not lawful to have a loan and a sale, nor two conditions in one sale." The reason for this is that a loan that brings a benefit to the lender is a means to usury. This prohibition can be avoided by making the guardian an agent for the questioner in transactions, provided that the fee is known and specified.