If the ball isn't sealed well enough to contain the CO2, then a CO2 discharger may not work because the gas will leak past the ball.
If the bore is rusty, or if the lead ball is badly encrusted, or if the ball pulling procedure isn't successful for some other reason like due to being just plain difficult, then there's still other options.
One such option is to remove the nipple and then find a grease zerk that will fit the nipple threads. Then pumping grease into the barrel with a grease gun should gradually lift the ball up and out of the barrel.
Another option would be to simply try to shoot the ball out. Since you're not sure how much powder was originally loaded into it, that should be done from a distance with a piece of string tied to the trigger. The gun should be tied down to a spare tire or such to help secure it so that it's always pointing in a safe direction.
A very small amount of powder can be placed under the nipple to help insure expulsion of the ball. That is, even if the ball was originally "dry balled" or loaded without any powder.
That's usually the easiest way to expel the ball in a gun that's in safe operating condition.
People often recommend against doing it that way with an old gun that they didn't load themselves because it could have been loaded with an excessive charge. But it's just as equally possible that it was loaded without any powder.
That's why it's recommended to shoot it out from a safe distance with a string tied to the trigger.
If the charge is first neutralized with water trying to use the ball puller then this method becomes
much more difficult to accomplish.
Assessing the condition of the barrel should be done beforehand, or at least show us some photos of the gun to let us see what it looks like.
Some balls are much more difficult to pull out than others by using a conventional ball puller which is basically just a threaded accessory with a screw that can be attached to the end of a ramrod or cleaning rod.
The last option that I can think of would be to unscrew and remove the breech plug after any charge is neutralized. Professional help is often necessary to get it out successfully without ruining the plug or the tang and then to reinstall it. After removal the ball can simply be pushed out from behind. If the gun was very valuable or in poor condition then this would an option to seriously consider.