what bet do bono and troy make? What warning does bono give troy?

I'm not sure about the bet, but Bono warns Troy to take responsibility for his own actions and to try to hold onto his wife. He explains that is Troy tries to have both women, he'll end up losing one of them anyway.

After looking further, I believe the bet was that Bono's wife (Lucille) would get the refrigerator she'd been asking for when Troy finished the fence for Rose. You think he was making a point that Lucille wasn't going to see her refrigerator for awhile? :-)