rear axle broke

We just paid a mechanic $720 to order a part from the chrysler dealership and put it on and my husband got 20 miles down the hwy with it and it broke again. The part was the rear cv axle or the rear axle. At first the mechanics thought that it might have been the tires was a little larger on the back than the front the reason why it jumped back out. We didnt know about the tires were different either. They tried putting the same size tires on the back and it still kept jumping out so he suggested that he wont charge us for the towing and he will tow it to the nearest Chrysler dealership because the couldnt fix it. We've already paid him $450 for the part and the remainder was labor but yet it still is not fixed! And now he is goin to tade it to the dealership and we got to pay that bill. If you ask me, I think if they couldnt fix it he should have left it alone and we could have taken it to the dealership the first time and I just dont believe that they cant fix it and they call themselves a business. Please help me! Any advice is better than none.