Is Fajr prayer valid if a man experiences seminal fluid discharge after the final ablution and before the prayer, knowing that the prayer time had ended when the presence of moisture was confirmed after the prayer?

What you did is correct. The discharge of a man's fluid from a woman's vagina after intercourse and ghusl only nullifies wudu', not ghusl. You were also correct in not paying attention to doubt after completing the prayer, because doubt after finishing an act of worship does not cause harm. This is due to the hardship and difficulty being lifted in the religion.