A tug-of-war ensued resulting in the trousers being torn down the middle. At which point a one-legged man (set up by them) comes into the shop and says something like "Just what I'm after"

A likely story but:

I remenber spending a lot of time finding out about the best pair of binoculars, for the best price, a friend having asked me to. I reported back and few weeks later he proudly showed me a monocular he had obtained (for half the price). In all my detailed research, I had overlooked the obvious fact that my friend had only one eye!

Absolutly true so perhaps the trouser story was also?