jesus lad what a story
A, Correct but the problem might have appeared after fitting and you driving it...
B, Not if the A garage said it was 100% sound, thats like getting a engine where stated 100% and thne start to open her up to make sure... they trusted the gearbox simples.
I'd go with getting a new box to be honest mate and then ask the garage to refund you the cost of the dodgy box and try and recover some of the cost to fit it to your car (if not all of it) (Id' go with a ne wbox from the same place, they'll be sure to get you one thats hopefully working this time!)
NOW, what way was the box bought? Did you get any warranty with it?
a