Torah Temimah cites Bava Kamma.
“And its fifths” — if he gave him the principal and then swore to him about the fifth, he pays a fifth upon a fifth until the remainder is less than a perutah.
Ribbuy of multiple fifths — successive oaths restart the process.
Each false oath about a remaining fifth restarts the process; only monetary claims enter the system. The plural “chamishitav” supports the successive-fifth structure.