What is the ruling on the fast of one who was about to commit zina (unlawful sexual intercourse) and masturbated at night to avoid it, and is his fast valid?

Masturbation is forbidden, but your fast is valid, because acts that invalidate the fast only do so during the daytime in Ramadan (definition), not at night. This is based on the Almighty’s saying: "It has been made lawful for you the night of the fast to approach your wives... And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night." The gravity of a sin increases depending on the time and place.