Soldato
Nice, and since you will be in the store how about helping yourself to a nice flatscreen tv or perhaps dip your hand in the cash register?
Edit: unless you don't actually mean how your post reads?
you could class it as being a bit cheeky i suppose but not theft, if the suit is still retailable you're just extending the 35 day return policy by buying a new one that DOES fit and taking back the one that doesnt on its receipt, saying that it doesnt
The company doesnt lose out anymore than if you took the actual newer suit back, it just goes back on the shelf to be sold to someone that it does fit

