Praise be to Allah.
We put this question to Shaykh ‘Abd al-Rahmaan ibn Jibreen (may Allaah preserve him) and he answered as follows:
If it was a small amount at that time (i.e., the time when the thing was made a waqf), but it has become a lot now, then I think that he should not be given all of that in return for his work. His work may be little, such as supervising and renting out and collecting rents and giving the money to a specific person or to a charitable committee. Based on that, it should be reduced to bring it in line with the salary of others (they should look and see, if another man were to be appointed to do the jobs that this supervisor does, how much would he be paid, then he should be given a similar amount).
And Allaah knows best.