What is the ruling on the validity of the funeral prayer if the worshipper intended to pray for a man, then it became clear to him that the deceased was a woman?

It is not a condition for the validity of the funeral prayer to know whether the deceased is male or female, because the purpose is to supplicate for the deceased. Some scholars have explicitly stated this, such as Ahmad al-Dardir, who mentioned that it does not harm if one does not know whether the deceased is male or female, and the worshipper may use masculine or feminine forms in the supplication. Similarly, Sheikh Ibn Uthaymeen said that it is permissible to use either the masculine or feminine pronoun in the supplication, depending on the intent. So whether one says, "O Allah, forgive him" or "O Allah, forgive her," the prayer is valid.