Tnai Batel U'Maaseh Kayam

There is a general rule that if someone does a maaseh with a tnai and doesn't formulate the tnai properly, we say that the tnai is batel and the maaseh is kayam. For example, if one performs a kiddushin under some condition, but doesn't formulate the condition with the proper mishpatei hatnaim (no kefel hatnai etc.), we say the tnai disappears and the kiddushin is chal. The question is, why should this be? If the husband did not want the kiddushin to take effect except under certain circumstances, shouldn't we say that there was no proper daas, and the kiddushin doesn't take effect.

The Shut HaRash (35:9 and 46) says that the entire concept of tnai is a chiddush. The idea that someone can make the challos of a maaseh conditional would never have existed if not for the fact that the torah allowed for it. Therefore, if the tnai is made improperly, the chiddush hasn't been executed, and m'maila the maaseh is chal.

However, the Rashba in Gittin 35b leaves this issue as a question.

I heard an explanation of this as follows (see as well Chazon Ish 50 and 53 in Gittin):

The Rosh holds that before the torah came along one could never have made a tnai in any maaseh. For example, can one imagine a tnai in a maaseh shechita. Likewise, a tnai in any maaseh makes no sense. A maaseh kiddushin should automatically take effect just like a maaseh shechitah.

However, the torah was mechadesh, that in certain areas of halacha this is not the case. One can make a tnai and prevent a maaseh kiddushin from being chal. Thus, if the tnai isn't made properly, the maaseh automatically results in its challos.

However, the Rashba disagrees. He holds that shechita and kiddushin are inherently different. By kiddushin, it isn't so much the maaseh kiddushin, but the daas for kiddushin that creates the challos. Thus, even though one can't create a tnai to stop a challos of shechitah, this isn't true for a maaseh kiddushin, or other maasim where the daas creates the challos. Thus, the chiddush of the torah merely was that if you make a tnai, this is how you should do it. The Rashba was, therefore, left with a difficulty. If the tnai is made improperly, why does that mean the maaseh is automatically chal?