Rashi on Leviticus 5:4/רש"י על ויקרא ה׳:ד׳

1בשפתים [OR IF A SOUL SWEAR, PRONOUNCING] WITH HIS LIPS [FOR HARM, OR FOR GOOD] — with his lips but not merely in his heart (Sifra, Vayikra Dibbura d'Chovah, Section 9 2; Shevuot 26b).

2להרע TO DO HARM to himself,

3או להטיב OR TO DO GOOD to himself, — as for instance, if he swears: “I shall eat” (which is להטיב), or, “I shall not eat” (להרע), “I shall sleep” (להטיב) or, “I shall not sleep” (להרע) (cf. Shevuot 27a).

4לכל אשר יבטא WHATSOEVER IT BE THAT A MAN PRONOUNCETH [WITH AN OATH] whatsoever it be: — this is intended to include the case where the oath refers to something that happened in the past (e.g., he swore, “I have eaten” etc. but he has not) (Sifra, Vayikra Dibbura d'Chovah, Section 9 8; Shevuot 26a).

5‎‏ונעלם ממנו AND IT BE HID FROM HIM (he forgot that he had taken such an oath) and consequently he violated his oath. — All these cases come under the law of “a sacrifice of higher or lesser value” (according to pecuniary conditions), as is set forth here (vv. 6—13): but an oath which involves the false repudiation of a claim to money does not come under the law of this sacrifice but under that of a guilt-offering (v. 25).