Pro and prae are somewhat similar in meaning and form. De Vaan isn't clear about whether they are related; he mentions Proto-Indo-European roots *proH or *pro, and *pre-h2i, respectively. Could it be some kind of ablaut?
In theory, *preh²i could be the locative singular of *proH, with e-grade ablaut (and possibly a different laryngeal?). I think this is what de Vaan is suggesting, if in a somewhat roundabout way.
