If I want to support an orphan

Q 3: An orphan boy who is now in the second year of prep school is being brought up by his maternal uncles. This boy used to receive a monthly award of one hundred and fifty Riyals during the period of his primary school and six hundred Riyals during the period of his prep school. The maternal uncles of the boy would take the money, mix it with their money, and provide the orphan with all his clothes, food, and notebooks that he needed. Now the orphan boy has attained a good degree of growth and maturity and his maternal uncles want to prepare his statement so that every thing is ready when he comes of age in a few years. The question is whether they should calculate all the awards that the young orphan received, meaning, the monthly amount of one hundred and fifty Riyals and the amount of six hundred Riyals and pay this to him in full as they do not know how much was spent from it and how much increased? It is important to mention that they did not record this before, so should they estimate the value or what should they do? Provide us with your beneficial answer please. May Allah reward you with the best and greatest reward and guide you to all goodness.

(Part No. 14; Page No. 239)  A: They should estimate the amount of money that they spent on the orphan and deduct it from his awards after calculating them. Then, they should pay the rest of the money to the boy. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.