Is the surplus taken by the Mukallaf (accountable person) for purchasing for himself lawful or unlawful, and what should he do if this surplus grows?

The 5,0 dinars that were taken fraudulently must be returned, along with half of the profits generated from investing that amount. Sheikh al-Islam Ibn Taymiyyah held the view that the growth resulting from usurped money that was put to work should be divided equally between the owner of the money and the worker, based on the ruling of Umar ibn al-Khattab in the story of Abu Musa al-Ash'ari's two sons.