Is it permissible for a person who was eligible for zakat, and whose income subsequently increased to a sufficient level, but who still needs to cover his children's tuition expenses, to use the zakat money he received, or must he return the zakat money and pay the tuition expenses from the additional amount he obtained?

Zakat (definition) enters into the possession of the poor person as soon as it is received. If he takes it while he is poor, he owns it, and if his poverty ceases, he is not obligated to return it, and he may dispose of it as he wishes. The expenses of children's education are considered an obligatory expenditure by some scholars, and whoever is unable to afford them is considered poor and eligible for zakat.