Generally speaking this is not a great idea. It won’t keep very long, so it depends on how long you plan on storing it. For later in the same day it is probably okay, but past that there can be issues if not stored in bacostatic solution.
Another issue is with reusing syringes and contaminating the dose with blood from when you register. If blood gets into the syringe it is really advisable not to inject the remainder of the dose later on regardless of how it is stored. And if you must use the rest of the dose, definitely transfer it to a new syringe prior to storage/use.
But really the answer is that there is no surefire safe way of storing part of a dose after you’ve injected some of it, at least if you plan on injecting it later.
Now, if you want to save the dose what you can do is plug the remainder instead of shooting it. Or waterlining it via the nose. Those are the only safe ways to use the rest of your dose, and storage isn’t as much a problem if you’re going to use it rectally or intranasally later on.