Is the fast valid for someone who stopped masturbating upon hearing the dawn adhan, but semen was discharged after stopping?

Masturbation is forbidden and requires repentance. If you stop masturbating at the time of the Fajr call to prayer and semen is discharged without your intention, then your fast is valid. This is similar to someone who had intercourse at night and then semen was discharged during the day; their fast is valid according to the jurists.