Is it permissible to take money from a person, which is a usurious loan from which he has not benefited, and to invest it using the Mudarabah system for a variable percentage of profit, knowing that a portion of the profits will be used to repay the installments of the loan?

There is no objection to dealing with a partner whose capital comes from a loan, where the effort and work are from you, and the capital is from him in exchange for an agreed-upon percentage of the profit. The prohibition of a forbidden loan pertains to the borrower's responsibility and does not prevent dealing with him in a permissible transaction. It is commendable to advise him to repent from the forbidden loan by feeling remorse, seeking forgiveness, and resolving not to return to it.