Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 189.
For a summary of the problem this post will look at, see Kidnapping Law, Part 1.
Here are the passages Fishbane looks at. The passages are in blue, and the interpolation (in Fishbane's eyes) is in red. The translation is from the New Revised Standard Version.
Exodus 21:16: Whoever kidnaps a person, whether that person has been sold or is still held in possession, shall be put to death.
Deuteronomy 24:7: If someone is caught kidnaping another Israelite, enslaving or selling the Israelite, then that kidnaper shall die. So you shall purge the evil from your midst.
Fishbane translates Deuteronomy 24:7 this way: "If a person will be found stealing one of his Israelite compatriots, treating him like an article of trade, and sells him, that thief shall die." Again, Fishbane treats ve as "and" rather than "or." He also translates hitamer as "treat as an article of trade" rather than what the NRSV has: "enslaving." My WTM Morphology on BibleWorks treats it as "to deal harshly." The word only appears in three places in the Hebrew Bible--here, Deuteronomy 21:14, and Psalm 129:7--and translations differ on what to do with it. The LXX tends to view it in terms of oppression.
According to Fishbane, Deuteronomy 24:7 has the same problem with Exodus 21:16 that he has, and he says Deuteronomy interprets the passage to mean that the kidnapper has been caught with his victim while he was in the process of selling him. Fishbane views Deuteronomy 24:7 as an interpretation of Exodus 21:16 because of similarities in vocabulary.
Incidentally, Deuteronomy 24:7 also has the order that Fishbane thinks Exodus 21:16 should have to be logical: kidnapping, the victim being in the kidnapper's possession, selling. Fishbane doesn't note this, however, for he treats Deuteronomy 24:7 as solely about merchandise.
Tomorrow, I may look at the Hebrew to determine similarities in vocabulary.