What is the ruling on money taken by a contractor who awarded a contracting project to another, in exchange for a percentage of the amount, while guaranteeing the quality of the work and its conformity to specifications?

What you did, agreeing with another contractor to carry out what you had contracted for, is called subcontracting. This is permissible provided that the project owner does not stipulate that you directly perform the work yourself, and that your personal involvement is not a critical consideration for him. If these conditions are met, then there is no harm, and this money is lawful.