I'd find a new garage firstly their is nothing and I mean nothing that can make the belt get stuck on anything

if the pulley sheared off. either they fitted a ****e part. or they fitted a good part so badly. They caused it to fook up. If the tensioner was so faulty that it could somehow cause the belt to throw itself off the pulley then they should have noticed that, when refitting the belt.
You've been bent over, shafted and charged for the privilege. Like I said. Find a decent garage. They sound like a bunch of ripoff cowboys.