If somebody said to Allah that He should not accept their dua if they steal item X and then they steal X, do they have to return item X for Allah to accept their dua? I understand that stealing is a serious sin and the item needs to be returned for Allah to forgive the person, but I am more asking whether the person needs to return the item specifically for their duas to be accepted.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What you mentioned is considered as supplication against one’s own self, and this is forbidden, as in the following Hadeeth:

“Do not invoke curses on yourself or on your children or on your possessions lest you should happen to do it at a moment when the supplications are accepted, and your supplication might be granted.” [Muslim]

As for the answer of the supplication against one’s own self, then it is hoped that it will not be answered, as some scholars said: The supplication against one’s own self is idle talk and one is not sinful for it.

Tafseer al-Baghawi reads:

“Zayd ibn Aslam said regarding idle talk in faith: it is the supplication of person against his own self, such as saying to a person: ‘May Allah blind my sight if I do not do such-and-such’; and ‘May Allah take my wealth away from me if I do not come to you tomorrow’, and he will say: ‘I will be a disbeliever if I do such-and-such’; all this is idle talk and Allah will not hold him accountable for it, and if Allah hastens their supplication, then He would hasten their punishment. Allah Says (what means): {And if Allah was to hasten for the people the evil [they invoke] as He hastens for them the good, their term would have been ended for them.} [Quran 10:11] Allah also Says (what means): {And man supplicates for evil as he supplicates for good, and man is ever hasty.} [Quran 17:11] [End of quote]

So that person should not renounce from supplicating Allah because of his supplication against himself, while being keen on clearing himself from responsibility and returning the stolen items to their owner.

Allah Knows best.