aha! Using my usual circular answers............a washed fabric would give you more wiggle room so that this kind of promise wouldn't fail.
A reason to offer to wash first, if of course the fabric could in fact be washed and look better or at least acceptable. More elastic that way, more forgiving.
As to fixing it once done, I have done did this myself, and it cost me $625 bucks and a friendship.
Mucho better to avoid offering, or to specify narrowly.