an illegal "condition"

whenever a Tnai is made which doesn't work due to a "technicality" such as Tnai Kaful, Tnai Kodem L'maaseh, etc. The question arises that although the technicality of the condition was illegal, but how can we say the action takes effect if he had no intention for this thing to happen if his conditions aren't met, in other words at the least ant condition tells about his intentions and they aren't met then the act was missing his consent.
i remember seeing once in the a T'shuva from the Rosh that we must say that it isn't clear to us from his actions that he wouldn't go ahead with this action if his conditions aren't met. So if it isn't clear to us from his actions then even if he wouldn't want the action to take effect its too bad, because the rule is that we only need him to have Daas to do the action and this he had. if however it is clear to us that his intentions were limited, then we apply the gemara with the person selling his property because he is making aliya, where if his trip is canceled so does his sale.
writers note: i know this is a discussion in the rishonim, i hope i am not mixing up a few opinions, please do your own research, i found the sources originally in the encyclopedia talmudis but i don't have it at home, lastly, yes i know tosfos discusses this. ut shabbos