Opinion Poll

Ok, we need some outside opinions on the following situation.
Two people go to buy a bar-b-que. Person A states that they will get the bar-b-que assembled for free. Person B states that if person A can get the bar-b-que assembled for free that they would pay person A $20.
In talking to the sales person, Person A asked if they could get the bar-b-que assembled. The sales person stated that the display bar-b-que was the last one in stock and if the two people wanted it, they could buy it. Hence, person A was getting an assembled bar-b-que free of charge, however, did not have to convince the store to do it. Should person B owe Person A the $20.
Please give us your opinion.