This sounds like a win-win for you and the retailer. You do indeed want to lead with a great headline because this is such a unique offer.
But at the same time, explain in the body copy what some of the exclusions or restrictions (if any) there may be.
Here are a few ideas. My wife a boy just came up to my office and I got to head out to Red Robin. I'll think of more later if I get a chance to stop by.
How about:
"Give Me Your Dirtiest Carpet. If it Doesn't Come Sparkling Clean, You Get Credit Toward New Carpet."
"If Your Carpet Doesn't Come Sparkling Clean You Can Get NEW Carpet."
"If You're Not Happy With Your Carpet Cleaning, Trade Your Cleaning For New Carpet."
"Local Carpet Cleaner Swears to Make Your Carpet Beautiful or You Get a $100 credit For Brand New Carpet."
"If Your Carpet Isn't Spotless After Our Thorough Cleaning, You Can Get New Carpet."
"If You're Thinking About New Carpet...I've Got the Perfect Solution For You."
Just a few thoughts in the spur of the moment.