פירוש על קידושין 8:5

The case of the yevamah (the woman awaiting levirate marriage) proves that just because one can be acquired through intercourse does not mean that she can be acquired through money. A woman awaiting levirate marriage is married to her brother-in-law when she has intercourse with him. He does not give her money.

The Talmud now rejects the difficulty raised through the case of the yevamah. A yevamah may not be acquired through money because she also cannot be acquired through a document. But a wife, who can be acquired through a document, should, so it is argued also be able to be acquired through money.
The Talmud now cites the proof from Deuteronomy again.
The problem is that according to the flow of the logic we should not need a verse—we have a logical argument drawn from the Hebrew maidservant from which we can deduce that a woman can be acquired as a wife through money.