38b
The gemoro though that a father who's nechosim are forbidden to his potential son in law, may never marry him his daughter, because he is giving him a free shifcha. But why can't he just sell his daughter for a large kesef kiddushin, like the gemoro says 31a he can sell things for far more than their value and this is not called giving hanooh.
Avrumi hersh , London england
You are suggesting that instead of marrying off his daughter, he should sell her as a slave, which is not what Rebbi Yitzchak is looking for. Another option you might mean is actually to marry off his daughter, but ask the groom to pay the sum of a slave. This might be Mutar, but Rebbi Yitzchak is talking about a normal case, and "charging" the price of a slave for Kidushin is not the regular situation.
I hope this helps!
Aharon Steiner