What are the causes of obsessive-compulsive disorder in financial matters, and to what extent does the Hadith about receiving good deeds on the Day of Judgment apply to such cases? When is money considered unlawful? And is there a jurisprudential rule that can be followed to avoid this obsession while safeguarding the rights of others?

Satan lies in wait for humanity, striving to mislead them. If he fails to tempt them through sins or acts of obedience, he approaches them through whispers (waswasah) to preoccupy their hearts and limbs. Waswasah comes from Satan, and engaging with it can lead to severe consequences. Therefore, one must cut off Satan's path by not paying attention to what he conjures up for you. Al-Suyuti stated that Satan whispers to those he despairs of misleading. Al-Nawawi said that the cure for waswasah is to seek refuge in Allah, turn away from thinking about it, and work to sever it by busying oneself with other matters. What you have mentioned falls under the category of waswasah. For the first issue, being certain just once was sufficient, and for the second issue, the manager's acceptance of the return of the box was enough. Waswasah has no reality; it is from Satan and should not be given attention. One should seek refuge in Allah from it. Allah Almighty says: "And if an evil suggestion comes to you from Satan, then seek refuge in Allah. Indeed, He is Hearing and Knowing." (Qur'an 7:200). Waswasah does not entail any legal ruling and is not to be considered, as the default is innocence. Fear Allah as much as you can, and do not pay attention to doubts, because you are afflicted with waswasah, and the cure for that is to turn away from it, busy yourself with other matters, and seek refuge in Allah.