Sunan an-Nasa'i, Book 35b, Hadith 4 (In-Book) ⓘ

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."

أَخْبَرَنَا مُحَمَّدٌ، قَالَ حَدَّثَنَا حِبَّانُ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ مَعْمَرٍ، عَنْ حَمَّادٍ، وَقَتَادَةَ، فِي رَجُلٍ قَالَ لِرَجُلٍ أَسْتَكْرِي مِنْكَ إِلَى مَكَّةَ بِكَذَا وَكَذَا فَإِنْ سِرْتُ شَهْرًا أَوْ كَذَا وَكَذَا شَيْئًا سَمَّاهُ فَلَكَ زِيَادَةُ كَذَا وَكَذَا فَلَمْ يَرَيَا بِهِ بَأْسًا وَكَرِهَا أَنْ يَقُولَ أَسْتَكْرِي مِنْكَ بِكَذَا وَكَذَا فَإِنْ سِرْتُ أَكْثَرَ مِنْ شَهْرٍ نَقَصْتُ مِنْ كِرَائِكَ كَذَا وَكَذَا‏.‏‏