"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." (Using translation from Nasāʾī 3860)  

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