Bava Metzia, Chapter 9, Mishnah 7. This Mishnah comes to answer the question: Does the lease payment that the tenant farmer must pay have to come specifically from the yield of the very field he leased, or may he bring it from elsewhere? The answer is brief: The payment must come from that field itself - whether this works out to the benefit of the field's owner or to the benefit of the tenant.
The text of the Mishnah:
"Hamekabel sadeh mechaveiro ba'aseres kor chitim lashanah" - a person serves as a tenant farmer and takes the field at a fixed rate, in exchange for ten kor (about one hundred bushels) of wheat a year. Based on this, the Mishnah discusses two opposite cases:
"Lakesah - nosen lo misochah" - it turns out that the wheat is defective. For some reason, the crop did not grow well, it developed mold, or the like. In this case, the tenant has the right to give the landowner the ten kor from that very field itself, even though the produce is unsuccessful.
"Hayu chiteha yafos, lo yomar lo hareini loke'ach min hashuk, ella nosen lo misochah" - it turns out that especially fine, award-winning wheat grew in the field. The tenant may not say: I obligated myself for one hundred bushels, and I will now buy one hundred bushels of grade A wheat in the market and give them to you. This is not possible at all.
The tenant must give the landowner the ten kor from the produce of the field itself, and if this is award-winning wheat - the landowner also receives that award-winning wheat.
In summary: The lease payment is drawn from the leased field and not from another source. If the crop is defective - he gives from it, and even though the wheat is inferior, he has fulfilled his obligation; if its wheat is beautiful - he gives from it, and he cannot exchange it with wheat he bought from the market.