Finally, you will have to decide yourself if you want to keep it or not and it depends a bit on your preferences and on how much oversized the shirt is.
I for myself prefer my base layer shirts to be skin tight since in colder weather I often wear two similar shirts on top of each other where every one qualifies as a base layer for warmer conditions. If they were sitting loosely, chances are high that you get some pleats on spots where you don't want ones, since they might cause chafing. Other people, especially such who think they have the one or other problem area or fat pad, might prefer their clothes sitting a bit more loosely to make those problem spots not point out too much. Again other people feel constricted by clothes sitting too tightly and prefer that even their base layer shirts to be some baggy style. Well, as I wrote in the beginning, it depends.
It depends also on the type of activity you plan to use it for. If you want it for example for running or stuff like that where you don't wear too much other clothes on top and typically no backpack, it's not too critical if the base layer is a bit crammed under the other layers. With a heavy backpack and some more layers of clothes this can be more of an issue.
Those are some rational thoughts. However, finally it boils to feeling. In my own experience, if some piece of gear does not feel quite right, i.e. it is "OK-ish" but a bit flawed in a way that I feel slightly uncomfortable about it, I tend to not use it if I have alternatives where I feel better. Such pieces have the tendency to diffuse deep down into the back of my wardrobe and get forgotten there over time – and this would be sad for a brand new shirt that cost some good money.
Therefore my final advice: if you feel not quite sure about it and have the possibility to exchange it for an equivalent but better fitting one at low or no additional costs, then get the better fitting one. You will be much more happy about it than about one where you think "I should have exchanged it" every time you wear it...