Ismael Bangoura and Nantes have been forced by Fifa's Dispute Resolution Chamber (DRC) to pay €4.5 million to Al Nasr as compensation for breach of contract.
The Guinean made the move to France in 2012, but it has now emerged that the switch was against terms agreed with the Dubai side.
As well as paying a hefty fine, Nantes will be unable to bring in any new faces for the next two transfer windows for the breach of contract, which came during the protected period.