From whom does the owner of the capital demand his money, which he lent to his partner - the Mudarib (investor/manager) - when a creditor denies having borrowed from the owner of the capital?

The Mudarib (investing partner) is not permitted to lend from the Mudarabah (investment) funds without explicit permission from the Rab-ul-Mal (owner of the capital), even if he has been granted absolute discretionary power. If he does so, he is considered to have transgressed, and the Rab-ul-Mal has the right to hold him liable and demand the amount of the loan from him. The Rab-ul-Mal also has the right to demand it from the borrower, because the action of the Mudarib in this instance is like the action of a usurper. By analogy with the ruling concerning a usurper who destroys the usurped property while it is in the possession of the donee, the one from whom the property was usurped may hold either of them liable as he wishes. If the Mudarib transgresses and settles a debt he owes using the Mudarabah funds, the Rab-ul-Mal may refer back to the one who received this debt.