Demai, Chapter Six, Mishnah Eight
In this mishnah we learn how someone who does tithe may enter into a partnership over land with someone who does not tithe, without having to tithe all of the produce for himself, thereby incurring a significant loss.
1) Two men who rented a field [for a share in the produce], or inherited [a field], or became partners in it: the one [who tithes] may say to the other [who does not tithe], You take the wheat which is in this place and I will take the wheat which is in that place. Or, You take the wine which is in this place and I will take the wine which is in that place.
2) But he may not say to him: You take the wheat and I will take the barley, or, You take the wine and I will take the oil.
Section one: There are three scenarios in this mishnah and in all three a person who does tithe has become a partner in a field with someone who does not tithe. For obvious reasons, the person who tithes does not want to end up paying for his partners tithes. What the one who tithes can do is declare that he will take the wheat from one place, or the wine from one place and then his partner will take the wheat or wine from the another place in the field. In this case we say that when the one who tithes receives his wheat or wine, he is getting only from his share and not from that of his partner.
Section two: However, what he cant do is separate the field by species or by type, saying that he will take the barley and his partner the wheat, or that he will take the oil and the other will take the wheat. The problem with this is that both of them own a share in all of the produce and therefore we cannot pretend that one person owns only the wheat and not the barley, or just the oil and not the wine.
It is interesting that the mishnah seems to want to allow people who tithe and dont tithe to enter into partnerships together, without causing a financial loss to the one who tithes. Perhaps we can detect a reality here in which people who were more religiously scrupulous than others had to live side by side. If so, the rabbis were searching for a way for a person to remain true to his ideals (one should tithe) without it costing him an exorbitant amount of money.