Torah Temimah cites Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, on 5:4).
Sefaria — Toras Kohanim (Sifra, Vayikra Dibbura DeChovah, on 5:4)
“When he swears” — he is liable for an oath (shevuah), and is not liable for a vow (neder).
The verse’s language of “tishava” limits the oleh ve-yored of this section to shevuah, excluding neder.