If a fasting person apostatizes and then returns to Islam later that day, is their fast considered valid?

This fast is not valid, and he must make it up, because apostasy invalidates it. Allah Almighty says: "And whoever disbelieves in faith—then his work has become null and void." Fasting is an act of worship whose condition is intention, and apostasy has invalidated it. He must refrain from eating for the rest of his day if he returns to Islam, because he broke his fast without a valid excuse.