The Economic System of Islam by Taqiuddin an-Nabhani

The Bait ul-Mal is the authority responsible for every income (revenue) or expense, which the Muslims are entitled to. Therefore, every property (Mal) that the Muslims are entitled to, and whose owner is not assigned, is assigned to the Bait ul-Mal, even if its owner as a category was assigned. Once the property was received, then by its receipt it is added to the rights of the Bait ul-Mal, whether the property actually entered into its possession or not, because the Bait ul- Mal is an authority and not just a place. And every right, which is due to be spent on the Muslims interests, is a right upon the Bait ul-Mal. If it was spent in its specified area then it becomes added to the expenses of the Bait ul-Mal, whether it left its hold or not. Because that which reached the governors of Muslims, or is spent by them, then the law of the Bait ul- Mal applies to it, whether as revenue or expenses.