Mass may or may not be constant, dependent upon the method used to vary the pressure; or more correctly the delta pressure.
We can change the delta P by changing the temperature, or changing the external pressure. In these cases, mass inside the balloon remains constant (assuming that the balloon is sealed).
However, from my youth I recall that a more common method to change the pressure inside a balloon is to connect it to a source of pressurized gas (commonly your lungs in a residential setting, but in commercial applications a tank of N2 or H2) and forcing gas into the balloon, thus adding mass.
For the "time sensitive" properties...take a balloon, blow it up, leave it for a day or two, then deflate. Compare to the original balloon. Blow it up again, and note how much easier it is this time.
