Torah Temimah cites Shevuos.
“A soul when it sins” — just as the oath of testimony incurs liability for the intentional as for the inadvertent, so too the deposit-oath.
Hekesh of mezid/shogeg liability — intentional and inadvertent equal.
Unlike ordinary offerings, shevuat ha-pikadon covers deliberate denial as well as mistaken denial. The asham is required in both cases.