The gemara says that one who would give a gett on condition that the gett be returned is a violation of the condition and action being in the same object i.e. the gett itself. Tosafos explains this issue that it is not just a technical condition in stipulation. The problem is that the condition to retain the gett is an inherent contradiction with the action of giving her the gett. Since the gett is only effective after the condition is fulfilled and at that point the gett is already back in his hands, it is impossible for the gett to be effective. But in a case where there is no contradiction between the action and the condition, the condition is allowed to be in the same object as the ma'aseh. The Beis Shmuel (E.H. 38:7) points out that the Rosh in Baba Basra disagrees with Tosafos. The Rosh asks on the case of giving an esrog to someone al m'nas - on the condition, that it be returned is a violation of t'nai u'ma'aseh b'davar ehcad. The language of "al m'nas" implies that the condition will make the action go into effect retroactively at the time that it was received, the condition does not in anyway interfere or contradict the action, yet the Rosh maintains that this would still be a violation of t'nai u'ma'aseh b'davar echad. It seems that the Rosh and Tosafos argue whether the concept of "t'nai u'ma'aseh b'davar echad" is a technicality in rules of tenaim like t'nai kaful, and t'nai kodem l'ma'aseh, or is is a practical consideration that the condition can't contradict the ma'aseh.
The B'nei Ahuva (from R' Yonasan Eibshitz Ishus 6:2) mentioned by R' akiva eiger (C.M. 241:6) asks on Rabbeinu Yerucham who seems to pasken that by a gett that is given on the condition to return the paper it is written on, is a violation of t'nai u'ma'aseh b'davar echad, so the condition is void and the gett is effective. Yet, when one gives an esrog on the condition that it be returned, the condition is binding and the receiver will only fulfill the mitzvah if the condition is met. Why is there a difference? R' Yonasan Eibshitz explains that really he holds like the Rosh that there is a violation of t'nai u'ma'aseh b'davar echad, even when there is no contradiction (since we pasken that al m'nas is k'meiachshav, so there won't be a contradiction). However, when one stipulates that the esrog must be returned the condition does not have to be fulfilled with the same esrog, rather the receiver can return a different esrog so long as it is just as good. But by the gett where the husband clearly is trying to cause anguish by demanding the paper back, the condition cannot be fulfilled with anything other than the paper the gett was written on and is therefore a problem of t'nai u'ma'aseh b'davar echad.
The rationale of R' Yonasan Eibshitz that when one borrows an esrog on condition to return it, can return another esrog, is based on the Rosh (3rd perek of Succha) who says that the original owner wants an esrog to be yotzei with (therefore returning money would not be sufficient). Therefore it should not matter whether it is the same esrog or another one that is just as nice that he can use to be yotzei with.