What is the ruling on the fast of one who had a wet dream, thinking it was pre-seminal fluid (madhy), but it turned out to be seminal fluid (maniy)?

The fast is valid, and purity from ritual impurity (janabah) is not a condition for its validity. As for the wetness after waking up, if the questioner is certain it is semen, then a ritual bath (ghusl) is obligatory. If he is certain it is not semen, then a ghusl is not obligatory. If he is unsure whether it is semen or not, then there is a detailed explanation.