What is the ruling of the Shari'ah on investing an orphan's money, and how are fixed assets handled upon his reaching the age of maturity, and how is Zakat paid on this money?

Renting out an orphan's apartment, spending on him, and saving money for him is a good deed. The saved money must be accounted for. If it reaches the nisab (85 grams of gold or 595 grams of silver), zakat becomes obligatory on it for all the past years, and its amount is one-quarter of one-tenth (2.5%). Upon his coming of age and demonstrating sound judgment, his money must be handed over to him in full, as Allah Almighty says: "And test the orphans until they reach the age of marriage. If you perceive in them sound judgment, then hand over to them their property."