Nothing good that’s for sure. 

However, it would have been nearly impossible for things to get to that point. Grimsley’s sister, Diana, was already very suspicious of her brother’s motives. Despite saying that he was going to be giving his baby up for adoption, he was cagey about the exact details of what agency he was working with and about the situation in general. If he hadn’t come to her first asking for help, Diana would have pressed her brother for information until he spilled. In this case, things would just progress roughly the same way they do in ‘canon’, with Grimsley going to Alola for about a year to help get his life in order while Hymnia is raised by her aunt and mother. 

In the very slight chance that Grimsley did manage to give her away he would very likely regret his actions enough to ask for help then. Champion Iris would likely get involved and, as in ‘canon’, the criminal ring would have been apprehended and Hymnia would have been tracked down and rescued. In this scenario, it’s possible that Grimsley might not ever get full custody of her, in which case Hymnia would be raised by Shauntal, or at least her father would have taken more time to help himself than the year he does in ‘canon.’ 

If somehow all of these scenarios didn’t happen and Hymnia was permanently sold off to fulfill her father’s bet however, then she likely would have suffered a terrible fate. Many people want to use mages for nefarious ends and, as a baby essentially sold on the black market, she could have ended up dead, raised to a be tool and used for her powers, or any other number of grim fates. In all likelihood however, this wouldn’t have happened. There are too many factors that prevented Grimsley from giving her up, from his own conscience to the interference of his family, so in the end anything that could have happened to her is just speculation. It’s something Hymnia thinks about on occasion, but since she avoided that fate she tends not to dwell on it.