P’ Matos is the source for the halachos of “mishpitei hatena’im” – when a person wants to make an act conditional on something else, there are specific rules that govern how to make such stipulations; these rules are modeled on the agreement made between the tribes of Reuvain and Gad and Moshe that their inheritance of Eiver haYarden was conditional on their participation in the war for the conquest of Eretz Yisrael.
One of the rules of tnei is that the act upon which one is imposing conditions must be able to be fulfilled by an agent (Kesubos 74 - interestingly, the Rambam never quotes this rule). The gemara in Nazir (11a) rejects an acceptance of nezirus conditional upon the person being able to drink wine because that tnei violates another principle – one cannot make a condition that undermines a Torah law, in this case, the law that a nazir by definition is not permitted to drink wine. Tosfos writes that before evaluating whether the condition in question undermined Torah law the gemara should have rejected the tnei for a more fundamental reason - nezirus cannot be fulfilled through a shliach!
Tosfos answers that although the laws of nezirus must be observed by the person who accepted upon him/herself the nezirus and not through an agent, at the conclusion of nezirus it is possible to send the required korbanos to the bais hamikdash through a shliach. In Tos’ view this element of shlichus fulfills the requirement needed to set up a tnai.
Though it solves the problem of tnai by nezirus, R’ Akiva Eiger points out that the gemara also has cases of a tnai in hilchos nedarim. There are no korbanos brought when a neder is completed, and the laws of neder must be observed by the person who makes the neder and not an agent. How then can a tnai be set up in such cases?