If it turns out the thief truly believes the property is theirs, this is called the claim of right, a common defense to larceny. The property converted must have come into the suspect's possession via a position of trust, commonly called a fiduciary relationship. For example, employees, parking lot attendants, dry cleaning services, auto shops, and bankers typically occupy such positions of trust. The actus reus elements of embezzlement are the same as larceny except that the element of taking is relaxed and the element of breach of trust is added. All that's necessary to prove breach of trust is that the property was handled in a manner inconsistent with the trust arrangement.
Embezzlement is a specific intent crime.
The requisite mental state is an intent to defraud and convert property. This mental state is sometimes proven by considering what happened after the act of larceny. If the person claims they intended to return the exact same property, it is false pretenses. If the person intended to return similar or identical property because they have already spent the profit, or covered up the losses in some way because of the physical impossibility of returning the original property, it is embezzlement. Persons who commit false pretenses do not have any lawful right to possession or any trust relationship.