"Then he shall be sold for his theft" — the Torah prescribes that a thief who cannot pay the required restitution is sold into servitude to raise the funds. But the Mekhilta adds a critical limitation: the sale must match the value of the theft exactly. The thief is sold "for his theft" — not for less than the stolen amount, and not for more.
Rabbi Yehudah elaborated on this principle. If the thief stole less than what he is worth as a laborer, he is not sold at all. The sale mechanism only activates when the theft exceeds the thief's ability to pay from his own resources. Selling a person whose labor value exceeds the theft would generate a surplus — profit beyond what justice requires — and the Torah does not permit this.
But if the thief stole more than his own worth as a laborer, the situation is different. The owner of the stolen property now has a choice: he can sell the thief into servitude (collecting whatever the thief's labor is worth), or he can write the thief a writ of emancipation (presumably forgoing the remaining balance).
This system carefully balances restitution with human dignity. The Torah permits selling a person to satisfy a debt, but it constrains the sale to prevent exploitation. You may not profit from someone else's servitude beyond what they actually owe. The thief pays what he stole — no more, no less.