Ref. No. 828/41-136
In the name of Allah the most gracious the most merciful
The answer to your question is as follows:
Paying grinding fee along with some amount of wheat is permissible if it not taken from the same grinded wheat. But if it is conditioned that the deduction would be from the same wheat grinded here then it would not be permissible and such a transaction is termed in a Hadith with ‘Qafeez e Tahhan’ which is an unlawful transaction in Shariah.
ولو دفع غزلا لینسجہ لہ بنصفہ ای بنصف الغزل او استاجر بغلا لیحمل طعامہ ببعضہ او ثورا لیطحن برہ ببعض دقیقہ فسدت فی الکل لانہ استاجر لجزء من عملہ والاصل فی ذلک نھیہ ﷺ والحیلۃ ان یفرز الاجر اولا او یسمی قفیزا بلاتعیین ثم یعطیہ قفیزا منہ فیجوز (ردالمحتار مطلب فی الاستئجار 6/57)
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband