Is money paid by craftsmen considered usury and unlawful, especially since the intermediary did not inform the workshop owner of this agreement, and is it permissible to return it to the craftsmen if it is unlawful?

It is permissible to take a commission provided that the payer is aware of it. If your two friends will pay it from their own money or wages, then your relative does not need to know, because you will not be taking anything from him.