Answer:
(Fatwa: 1675/1128/H=1433)
If the person does not commit any illegal work and also it is not his responsibility to provide you scholarship, so he fixes any amount in advance as per the compensation of his hard work instead of 20% then it is allowable, otherwise not.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
