If you have made a neder to use this particular food only on shabbat, then you would have to keep the neder. However, if you have set it aside for shabbat, then you have only determined that you will use it for that shabbat. Once shabbat is over, the food is still usable. As an example you can see Shamai HaZakein Buying for Shabbos All Week where Shamai Hazakein (Beitzah 16a) set aside food for shabbat and replaced it if he found better food and ate it during the week.
Similarly, once food is left overs it is now of a lower quality than what would be made fresh the next week and can be eaten during the week (like Shamai did).