Torah Temimah cites Shevuos 35a.
Mishnah: If one says to witnesses, “I adjure you that if you do not come and testify that so-and-so said he would give me two hundred zuz and did not give,” they are exempt — for they are liable only for a monetary claim, as with a deposit.
The Mishnah limits shevu’as ha-eidus to claims that create a collectible monetary obligation. Pure verbal undertakings without monetary consequence fall outside the verse’s liability structure.