@Norma137 Well, your reviews from past hosts indicate that you're a good guest, neat and clean. Perhaps that paint spot was already there and the host didn't notice it before. Or maybe it's not paint- did anyone in your group apply nail polish perhaps? Or something else that might have left that spot?
Even if you or someone in your party did leave that spot, it does seem over the top to insist you pay for a new couch because there's a dime-sized spot- there's likely some way to clean it without damaging the finish and it was good of you to offer to pay for that.
I'd just dispute the charge. Call Airbnb if the host gets nasty about it, ask them to take a look at your past reviews and the message you sent offering to pay for cleaning. The host would have to submit proof to Airbnb of the damage and cleaning costs to charge you anyway if you dispute that it is your fault, so it may not come to anything but empty threats.