What is the ruling on the prayer of one from whom madhy (prostatic fluid) was discharged after ghusl (ritual bath) and before prayer, and he only became aware of it after finishing the prayer?

If you find wetness after prayer, and you are certain or highly assume that it appeared after performing ablution, then you must repeat your ablution and your prayer. However, if you are unsure whether it appeared before or after ablution, then you are not required to repeat them, because the default state is the continuation of purity. You are also not required to repeat the prayer due to impurity if you were unaware of it.