Celtic never put a clause in the contract that was violating their rule. It was between the clubs, but Celtic had no power to enforce them to not play Bangoura. Celtic could have asked them to not play him, but Elfsborg would only have done it out of respect rather than obligation. By forcing Atletico to pay you to play him, you're preventing them from playing him, which is against their rules.
Then I really don't see why the clause was put in the loan deal? Oh well we may not even have to face atletico anyway.