(The cycle moves this week from Gittin (about divorces) to Kiddushin
(about marriages). I wonder why that order and not the other.)
The talmud discusses valid and invalid ways of acquiring a wife, and
during the discussion takes a side path into acquiring slaves.
The sons of Rabbi Chuna ben Abin bought a slave for copper coins
(that is, the price was specified in coins). However, they did
not have coins with them, so they left a silver ingot as a pledge.
The slave's value then increased and the owner sought to cancel
the arrangement. Rav Ammi ruled that the owner could do so,
because the buyers had not paid coins and the ingot was not a
payment. (8b)