Is it permissible to take money from people and invest it for them in contracting and construction through a third party, while receiving a portion of the profits in exchange for guaranteeing the funds and bearing full responsibility, without the knowledge of the fund owners about the operating party?

It is not permissible for the Mudarib (investing partner) to profit without the knowledge of the capital provider. If it is with their knowledge, then it is permissible for him to take a commission as a broker, or as a Mudarib who pays the capital to another Mudarib. This is permissible according to the Hanafis, contrary to the majority of scholars. The profit is for the worker and the capital provider only, and not for anyone else.

Guaranteeing the Mudarib invalidates the Mudarabah (partnership). There is no guarantee upon him except in cases of transgression or negligence, and it is not permissible to take a fee for guaranteeing.

Writing checks to document the receipt of money is permissible, but the Mudarib is not liable for losses except in cases of transgression or negligence. A known percentage of the profit must be agreed upon in Mudarabah or partnership, and a fixed amount or a percentage of the capital is not valid.