What is the ruling on someone who stole something, then falsely swore by disbelief that he did not steal it, without intending disbelief by that?

Swearing by disbelief (kufr) is forbidden. The one who swears by it does not become a disbeliever unless he intended disbelief. He must repent and seek forgiveness, and return the stolen item to its owners. Swearing by anything other than the names and attributes of Allah does not constitute a valid oath, and the swearer is only required to seek forgiveness and repent.