You might be right but here is why I think he asked for $80 instead of the $60 which would have got him dead even. I think he was planning to quit after that set whether it is was $60 or $80, and indeed in the original post it says that B wanted to play "one more set" so apparently that was intended to be the agreed final set regardless of outcome. As said in my previous post I think he just wanted out of what he knew was a bad game and was taking a long shot on finally winning a set and getting out of that game without it costing him ("surely I can't lose four out of four, surely he can't win every single set forever, surely I am due to finally win one"). Maybe he had no intention of paying off either way so in essence he could win an extra $20 but couldn't lose any more if he wasn't going to pay anyway.
That said, whether he intended to pay if he lost that set or not, the biggest factor I see in making it an $80 set instead of $60 is that it lets him quit financial winner if he pulled off the win that set. It was a way to save his pride, give him some bragging rights, and humiliate his opponent at the same time. He had nothing to lose and everything to gain. If he lost that set too, what did it matter? From a pride and humiliation/bragging standpoint there isn't much difference between losing 3 out of 3 sets or losing 4 out of 4 sets. You clearly got rolled either way and there isn't any way to sugar coat that to yourself or to anyone else. But if he managed to win that last set and quit $20 up, he was the winner. His pride is intact. He gets to rub it in his opponents face that he beat him. He gets to brag to others that he beat A gambling, and he did. There is no way to argue against the fact that he won, because the money is what counted.
He wouldn't have had any of that if he played for $60 and managed to win that final set. The only way it could have been perceived or argued in that case is that he was in a bad game getting rolled, managed to win a single double or nothing set to get even, and had already arranged to quit after that set because he knew how out matched he was and wanted out and got lucky to be getting out without having lost any money. But for a measly extra $20, it gave him the opportunity to instead have the ego, bragging, and humiliation for the opponent rights that come with being winner. If he had lost it wouldn't have looked any worse at an additional $80 instead of $60, so he had nothing to lose and lots to gain by tacking on that additional $20 so he had a chance of coming out winner instead of things being so humiliating and looking so bad if he had lost or come out even.