I once heard about an abbasid ruler that was born because his father made a bet with a woman that whoever loses the game of chess will walk naked in the house and then the woman lost and got pregnant of the abbasid ruler. I don’t know for sure but I think it’s al Ma’moon. Can you confirm this?

Yes, but it is the other way.
When she won, she asked her husband Haroon to sleep with a servant, and the servant became pregnant with Mamoon.
However, it is a story.