There is an interesting chakira[1] regarding the timing of the mitzva of tzitzit. Is the mitzva to attach the tzitzit to the tallit katan (the garment) before it is put on?[2] Alternatively, does the mitzva to put on the tzitzit actually begin, at least in theory, only after one puts on the garment?[3] In any event, since you knowingly put on a garment that had no tzitzit without intention to immediately attach tzitzit (albeit with the intention to take it off relatively soon thereafter), your question is a good one. Nevertheless, there are two reasons why putting on the garment without tzitzit was fine in your case.
The gemara[4] infers from the commandment to attach tzitzit to “the four corners of your (in the singular) garment”[5] that one is exempt from putting tzitzit on a borrowed garment. (After borrowing it for 30 days, a Rabbinic obligation kicks in because it appears that it is his own garment). Although the Shulchan Aruch rules that when one borrows a tzitzit garment, he should make a beracha before putting it on (i.e., the obligation applies), that is true when the garment and the tzitzit are complete and ready for the mitzva. In that case, we assume that the owner is willing to transfer ownership to the borrower (on the condition that it will be returned) so that the latter can fulfill the mitzva.[6] Clearly, however, when it is not practical to attach the tzitzit, neither you nor the storeowner is interested in your acquiring ownership of the tallit katan, and the obligation of tzitzit therefore does not apply.
A similar situation arises when one uses a shul-owned tallit for an aliya or the like. Although classical sources[7] indicate that in such a situation, one should recite a beracha before putting the tallit on, common practice is not to do so, apparently because the congregation would have to wait too long as the person makes the beracha and puts it on in the appropriate way. Therefore, one leaves the tallit in the congregation’s possession and does not make a beracha on it. (We will not discuss in this context what the optimal practice is.)
There is another reason why you were not required to have tzitzit on the garment you put on, and this one would apply even if you were trying on a tallit katan after buying it. First, we must address another halachic issue affecting clothing. The mishna[8] says that it is permissible[9] for one to put on a garment containing sha’atnez in the course of selling it (to a non-Jew) on the condition that he does not intend at the time to also benefit from it as a garment. The Shulchan Aruch[10] accepts this ruling. The commentaries[11] explain that the wearing of the garment is considered a davar she’eino mitkaven (a prohibited action that one does without intention) in that he is not interested in wearing the garment per se.[12] Tosafot[13] says that the same leniency applies to wearing a tallit katan without tzitzit for the purpose of selling it. The requirement of tzitzit applies only to garments that are being worn for the purpose of immediate benefit from wearing them. The same logic applies to one who is trying on a tallit katan momentarily for size.[14]
Therefore, what you did – as many others do – was certainly alright.