How can a student of knowledge repay the kindness of his teachers, and are there other ways besides supplication to return this favor? And what should he do with a teacher who does not accept payment for memorizing the Qur’an and teaching Tajweed?

It is sufficient for you to show appreciation to those in charge of the Fatwa Center and other sheikhs by supplicating for them in their absence and speaking well of the website and the sheikhs. The Prophet, peace and blessings be upon him, said: "Whoever has a favor done for him and says to the doer: 'May Allah reward you with good,' he has indeed expressed the utmost praise." And a Muslim's supplication for his brother in his absence is answered, and he receives the like of what he supplicated for. It is obligatory to combine supplication and praise for those who do good, as in the hadith of the Muhajireen and Ansar. If an unburdensome gift is easy to provide, it is better.