Is it permissible for the supervisor of mosques in a charitable organization to build the mosque himself instead of a contractor, and then donate 30% of his profit to the organization, knowing that contractors refuse to work for the approved amount, and this agreement is made with the approval of the head of the organization?

If the head of the institution assigned this work to you without favoritism, and the building materials, tools, and laborers are from your side, then there is no harm in that, provided that the contract specifies the price and describes the construction in a way that removes ambiguity, and that it is subject to the same rulings as a manufacturing (istisna') or forward-sale (salam) contract.

However, if you do not provide the building materials, tools, and laborers, but rather the employer provides them, then you are an employee (ajeer), and the contract in this case must meet the conditions for a valid employment (ijara) contract, such as specifying the wage and duration, and that the benefit be known by description or custom.