contract cancellation complaint- rosebank mall store

i went into the rosebank mall mtn on the 14/11/2024 to buy two phones. on the 15/11/2024 i returned one of the phones as i couldnt afford it. now the store doesnt want to cancel my contract they keep saying i must come and take it back and they saying i have to pay a cancellation fee whereas i returned the phone to the store accordingly based on retentions department advise that it was still within the cooling off period. the store doesnt want to help me at all and retentions department keep reverting me back to them. i am being billed for something i dont have on me.