No. There is no way to undo it, that would defeat the entire purpose of the infert philter. The pet is infertile exactly so her value is retained
Project pets that are sold are almost always made infertile by their creator so that other people cannot buy those pets and then undercut the project-maker's prices, thus keeping the value of the project.

Because doing projects takes a lot of time and effort and money.
Like. a lot of time. It can take a year or more of dedication to do something like complete a 6v or max stat project


For example, people not inferting their 4v emperor's dragon zolnixi projects way back when is the reason ED nixi are now worth scraps compared to pretty much... every other Majestic theme in existence.


If you don't want to buy in inferted pet, check the 'include only pets that are fertile' toggle in the advanced search.