What is the ruling on giving the agricultural crop to a person to harvest it, on the condition that the crop be divided between them by mutual agreement, and upon whom is the zakat of the crop due in this case?

The fundamental principle concerning conditions is their validity, unless they make a forbidden act permissible or a permissible act forbidden. Based on this, if someone gives their crop to another to harvest it, with the agreement that the harvester receives a quarter or half of it, there is no harm. Zakat is obligatory on the owner of the crop for the entire yield. The one who undertook the harvest is not liable for zakat because he is an employee (اجير), and the threshold (nisab) is not reduced by the expenses of harvesting or similar costs.