Commentary for Kiddushin 14:10

"Rav Yosef said: The [silks] need evaluation". [The Talmud] means [it needs to be evaluated] before kiddushin, just as Rashi explained. Problem: Later on at Kiddushin 48a, there is a baraita about "Be betrothed to me with this contract... and Hakhaimim say: Evaluate the paper; if it is worth a perutah then she is betrothed [and if not, she is not]". This implies that they evaluate the paper after kiddushin, even though originally the evaluated worth of the paper was unknown! Solution: [According to] the version [of the mahloket Rabbah-Rav Yosef here] that says that they both agree on "a small amount (kol de-hu)", [the case in Kiddushin 48a with the contract] is dealing with when he said to her, "[The contract] is [worth] a small amount". But [according to] the version [of the mahloket] where they also disagree on "a small amount", this is what it means: If he wants to betroth her with a contract, they should evaluate the contract beforehand—"if it is worth a perutah, she is betrothed" i.e. if they evaluated it beforehand—to exclude the one who says she is betrothed because it's a contract, even if it is not worth a perutah.

The issue is whether the woman must know exactly how much the silk is worth. Assumedly there is no question that it is worth at least a perutah, which is a very small amount.

The Talmud narrows down the scope of Rabbah and R. Joseph’s disagreement. If she explicitly agrees to be betrothed for whatever it is worth, then she is betrothed. She has a right to agree to this arrangement.
If he lies to her, obviously she is not betrothed. That would be a false acquisition
The issue is if he tells her what it is worth and his assessment turns out to be correct. According to Rabbah, the woman does not need to know beforehand that his assessment is verified. As long as what he says turns out to be true, she is betrothed.
But to R. Joseph, at the moment of betrothal she is uncertain. She is agreeing to something whose value she can’t know. Therefore, her agreement to be betrothed is not really a full agreement. The silk would need to be evaluated beforehand. Here we can see an important concept in the rules of kiddushin—the woman needs to have acquiesced to the proposal and we need to be sure that she knew what she was agreeing to.