´It was narrated from Hammad and Qatadah, concerning a man who said to another man:` "I will lease (something) from you until I reach Makkah for such and such a payment, and if I travel for a month or such and such -something that he named- I will give you such and such in addition." They did not see anything wrong with that, but they did not like it if he said: "If I travel for more than a month I will deduct such and such from your lease."
Hadith Referenceسنن نسائي / كتاب المزارعة / 3891
Hadith Gradingالألبانی:صحيح الإسناد مقطوع | زبیر علی زئی:إسناده صحيح
1؎: If, due to the slowness of the ride, it becomes necessary to do so, then I will do so. 2؎: Covering the specified distance earlier than the appointed time due to speed, and thus receiving extra wages as a reward and honor, is why both have deemed it permissible. However, reducing the fare due to slow travel is akin to oppression and usurping the right of the other, which is why both have considered this second scenario to be disliked (makruh).
Explanation & Benefits
Hafiz Muhammad Ameen
The intent here is that if the ride goes faster and takes less time, then I will give you more money, and if the ride does not go faster and takes more time, then I will give you less fare. The first scenario is permissible because it is in the form of giving a reward, and it is evident that giving a reward is permissible. The second scenario is prohibited because it involves injustice to the owner of the ride: not only did it take more time, but the fare is also reduced. And injustice is not permissible.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 3891