Torah Temimah cites Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, Section 10).
Sefaria — Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, Section 10)
“For a chatas” — so that the money set aside shall be designated for the sake of a chatas.
Diyuk of purpose designation — “le-chatas” requires that the funds be consecrated specifically as chatas money.
When a poor person sets aside coins for this flour offering, those coins are chatas-money and cannot be freely redirected to another purpose. The designation created by “le-chatas” binds the money to the chatas obligation.