Reference for Sanhedrin 29:6

"If so, then it would be property that has capacity for mortgage [i.e. the wife, that is bought along with property that doesn't, i.e. the maneh, which doesn't work]!" Rashi's explanation: "Because humans are compared to land, as it is written, 'You will inherit them [Canaanite slaves] as an inheritance holding [a phrase normally applied to land]'. Problem: This verse is written about a slave, but a freeman is not compared [to land] except only in [the case of one who vowed to bring to the Temple by saying] "My own value is upon me", since [that person] is evaluated like a slave as we see in the first chapter of Sanhedrin (15b), and a slave [being similar to land] is anyway subject to a dispute in the first chapter of Baba Kama (12a) whether they are similar to land or moveable property! Solution: [Rav Ashi's objection] is not specifically [based on the fact that people] have capacity for mortgage, but even if they don't have capacity for mortgage, there is no case where moveables can be acquired "on the back of" moveables [unlike land, for which you can acquire all the moveables "on its back"], since [the idea of] "on its back" is not relevant for property that doesn't have capacity for mortgage. Alternative solution: This is what he meant to say: You're forced to say, if a wife is acquired "on the back of" moveables, a field could also be acquired "on the back of" moveables, for we derive the acquisition of money for a wife from the field [of Efron] from [the ג״ש of] "purchase" "purchase", and if a field can't be acquired [in this way], from where would you derive it for a wife!?