Does the fast become invalidated and sin be incurred by one who deliberately experiences a wet dream during the daytime in Ramadan, when he was able to stop the dream but indulged in it until emission occurred?

The emission of semen merely by thinking does not invalidate the fast, whereas its emission through an action, such as masturbation, does invalidate it. Deliberate thinking about sexual matters while awake incurs sin upon the doer, but fleeting thoughts and whispers (waswasa) that do not settle in the soul are forgiven. One must repent to Allah and refrain from indulging in forbidden thoughts.