Rashi on Leviticus 5:24/רש"י על ויקרא ה׳:כ״ד

1בראשו — This means the principal — the capital (ראש) money.

2וחמשתיו [AND HE SHALL ADD] THE FIFTH PART [MORE THERETO) — By using the plural וחמשתיו the Torah includes in this law of restitution the many additional fifths possible in respect to one principal — that if he denies the fifth (i. e. he asserts that he has repaid both capital and fifth, but has not really paid the latter, for which a claim is now made against him) and takes an oath that he has paid it, but afterwards admits the claim, then he must now bring (pay) a fifth in addition to this fifth (a fifth of the original fifth which now has become the קרן in addition to it), and so he keeps on adding a fifth to the original fifth until the principal about which he takes an oath becomes less in value than a P’rutah (Sifra, Vayikra Dibbura d'Chovah, Section 13 12; Bava Kamma 103a).

3לאשר הוא לו [AND HE SHALL ADD THE FIFTH PART MORE THERETO AND GIVE IT] UNTO HIM TO WHOM IT APPERTAINETH — i. e. to him whose is the money (Sifra, Vayikra Dibbura d'Chovah, Section 13 12; Bava Kamma 108; also Bava Kamma 103a).