Albert Barnes Commentary 2 Samuel 12:6

"and he shall restore the lamb fourfold, because he did this thing, and because he had no pity." — 2 Samuel 12:6 (ASV)

Fourfold - The exact number prescribed by the Law (see the marginal references), and acted upon by Zacchaeus. The Septuagint has “sevenfold,” as in Proverbs 6:31.