Peah, chapter 8, mishnah 6. The mishnah establishes that this measure - the rule that a single poor person is to be given certain amounts - is "amurah b'chohanim uv'leviyim uv'yisre'elim" - stated with regard to kohanim, Levites and Israelites. In other words, these laws apply to a kohen, to a Levite and to an Israelite alike. What does the mishnah mean by this statement?
Two ways of understanding it:
From the side of the giver: when the giver, the farmer, is a kohen, a Levite or an Israelite, these laws apply to him. Even though the kohen and the Levite are not farmers with large landholdings, and sometimes own no more than a small field, since their livelihood comes mainly from Terumah and tithes, nevertheless the very same obligation rests upon them: to give the poor person enough to satisfy him, enough to make him full.
From the side of the recipient: some understand the opposite, that the kohen, the Levite and the Israelite mentioned here are the recipients. Even though the kohen and the Levite receive Terumah and tithes, if they are poor and receive the tithe of the poor - which they are permitted to accept - they too receive enough to satisfy them, enough to make them full.
One who wishes to benefit his poor relatives:
A person who has poor relatives and wants to make sure that they receive may do so, but within limits. The mishnah uses a delicate expression for the fact that he wishes to save his tithe of the poor and set it aside for his relatives: he takes half of the tithe of the poor that he has and designates it for his relatives, while the other half is given to the poor at large.
And what is the law when he has only a small amount?
If all that he has available for the tithe or for the poor person is a small amount, not enough to give a poor person his fill, he places it before the poor and they divide it among themselves. Even though he does not have enough to satisfy them, he has no choice, for he has no more than what he has, and the poor will divide the small amount among themselves.