What is the ruling on the unexpected increase in the contractor's profit, who built the mosque, which resulted from his receiving donations during the work, knowing that the estimated cost was 15 million, while the actual cost became 8 million?

Excess donations beyond the agreed-upon cost of building the mosque with the contractor are considered a trust (amanah (definition)) in the contractor's possession, and he must return them to their owners, unless they authorize him to spend them on another mosque or something similar.

However, if the contract with the contractor is an istisna' (manufacturing/construction) contract, meaning he builds the mosque for a specific sum, which is 15 million, then he is only entitled to this amount. Any donations exceeding it are a trust that must be returned. But if the contract entails the contractor's supervision and follow-up without specifying his fee, then he is only entitled to a customary wage (ujrat al-mithl), and any amount exceeding the cost and donations must be returned to the donors.

Therefore, the ruling differs depending on whether the contract is an istisna' contract, which entitles the contractor to a profit from the specified amount, or if he is an agent who supervises without volunteering, in which case he is only entitled to a customary wage, and the surplus is returned to the donors.