If someone made a promise to Allah in a dua to never masturbate again, and he later breaks that promise, what is required of him?
In the name of Allah the Inspirer of truth.
From the apparent wording of your question it seems as though this was not an oath, hence, what is required is sincere repentance to Allah and a resolution not to do it again. No kaffara is required.
However, if any of the following or similar expression were used when the du’a was made: “By Allah I will not do this again, or “I swear by Allah….”, or “I make this action haram upon myself…”, then even though the action is already haram it will be considered an oath as well. By committing that unlawful act the person will then be sinful as well as have broken an oath, so have to pay the kaffara. (See al-Durr al-Mukhtar 3:46)
And Allah knows best.
Mufti Abdurrahman ibn Yusuf