Interesting, and they'll disappear so quick you'll swear you seen a flying rug!
I have heard of this strategy. It sounds rock solid, but what happens if you spill something on the rug? Are they going to tempt you into taking your money now, and give you a rough time later? You already mentioned the guy was somewhat assembled as a charmer - but can you hand out rugs left and right knowing that one day you may have to refund every penny back to that customer?
The fact of the matter is, people in the rug business such as myself are in just that: a business. They need to walk in and out of their stores everyday with one thing on their mind- the ability to have a sustainable business which is both profitable and feasible long term. I'm not surprised by this "take your money give it back later scheme." If this store is new, he may have debt to pay off. What happens in two years when you see this guy's hosting a going out of business sale? Or maybe the when you go to turn in your rug, you fall in love with a new carpet that's not as good quality, but the exchange ends up being a "wash out".
My advise to you is this: If you're seriously interested in returning a carpet within any period of time, you shouldn't be purchasing it. Yes, prices and labor costs are going up for Oriental Rugs in countries such as Iran and exchange rates don't help..... Replacement values are going up, but enough that this seller can bank on his entire business to revolve around this exchange policy? I don't know about that. Think about it - if you buy a rug from him today, and you see something you love more tomorrow at another store, he just ends up getting used with no monetary compensation.
This is what I would do if I were in your position and seriously considering a carpet. First I'd be very skeptical, but secondly, if I absolutely had to have the rug, I would ask him to personally guarantee any promises, and should there be a problem, you both consent to the hiring of an ADR. Have this all spelled out in writing, especially any issues revolving around washing and shampooing, if the colors run, he's not going to take the piece back. Be VERY careful about what you sign. There's tons of small print in a deal like that I'm sure.