Early cancelation fee for what ...

I've cancelled my contract with mondo/mtn October 2021 and returned the device within 5 days...and received confirmation that it was received...surprisingly now I tried to apply for something somewhere boom it was decline because of mondo/mtn as they claim that I owe them R3300 for early cancelation...how possible is that...kindly remove my name on Itc , on your legal and collection list asap ...i really curse the day I app**** for a contract with mondo