Kesubos 74a - Conditions and Agent for Chalitzah

The gemara says that one is not able to make a stipulation that will prevent the chalitzah from taking effect, because we learn out from t'nai b'nei gad u'bneir reuven, that only that only a ma'aseh that can be done through an agent can one make a t'nai on it, but if it can't be done through an agent, one cannot make a stipulation on it. Tosafos explains the connection between making an agent and the ability to limit the action through a stipulation. Any action that is totally within one's control that he can even set up an agent to carry it out, it is also under his control to be able to make a stipulation to prevent the action from taking effect. But, an action that is not under one's control to set up an agent, but rather must do it himself, he cannot impose a stipulation to prevent the effect of that action. Therefore, since chalitzah is an act that cannot be done through an agent, one cannot impose a stipulation to prevent the chalitzah.

Rashi seems a bit strange. Rashi says that chalitzah cannot be done by an agent means, "one cannot say to his agent, if she gives you 200 zuz then do chalitzah, therefore it does not qualify as a stipulation to ruin the chalitzah". Rashi clearly understands that the inability to set up an agent on the act of chalitzah, is not enough of a reason to disallow a stipulation on chalitzah. The gemara is not focusing on a weakness in the act, rather on a weakness in the stipulation. Meaning, any situation where you can't set up an agent with a stipulation, you can't make a stipulation at all. v'dok!