Leviticus 5:11

But if he be not able. — The benign consideration for the poor, and the desire not to mulct them too heavily for their frailties, are here still more evinced in the statute before us. If anyone is so impoverished that the offering of two birds would press too heavily upon him, he might bring the tenth part of an ephah of fine flour, a little less than half a gallon.

For it is a sin offering. — That is, because it is a sin offering, and not a Minchah or meat offering (see Leviticus 2:1), therefore it shall have no oil or frankincense, otherwise its distinguishing features as such would be destroyed.