My question wasn't about finding some sort of loophole, just if it was a valid use of a coupon... I would have left it in MIA AC but left in a hurry and forgot about it until I looked at my documents when boarding the plane

I thought my question was clear, just out of curiosity, if anyone that found the coupon at DFW would have been able to use it or not. Apparently, there is a chance someone might have done it by exchanging it, which is better than a total waste.