Okay wait, maybe I just misread it on the first place, but how they will reversing the txn? or you mean you will send it back to them or to their wallet?
Edit 2: Emailed them and they are now reversing the txn.
OP withdrawn CAD in his bank account and the exchange sent OP higher than he requested. The case was something like this as far as I remember. May be after contacting with the exchange, exchange are trying to go through kind of charge back with legal procedure.
What you misread is you thought txn = crypto transaction.