Musannaf Ibn Abi Shayba 13:2513

Abu Bakr narrated to us, he said: Abdullah bin Numayr narrated to us, from ‘Ata’, regarding a man who hires out [transport for] food to a land for a [fixed] measure: if it increases, it is for them, and if it decreases, it is upon them. He said: If the hired parties are satisfied with that and acknowledge it, then there is no harm in it.