Why did she steal the money? $80 and $50 is a lot of money. My grandchildren at that age would sometimes pick up small change that was around the house or one time took a couple of dollar bills out of my wallet. They didn't understand about ownership. They took it just because it was money and they wanted some money. They didn't try to hide the fact that they took it. I did not consider the theft as a big deal. It was just a matter of teaching them that the money belongs to me and that they have to ask if they want some.
How you treat this would depend on the circumstances, why she took it, how you learned that she'd taken it, how well she understands that it's wrong, and what her reaction was when you found out. Also, did you get the money back? Was the money laying out so that it was a big temptation or did she have to get into your purse to get it? (I do wonder why, since she'd taken money before, you had that much money available to her.)
I would not cancel the party. Because you've told other people not having the party would be difficult for you and others. I try not to "punish" myself when I mete out discipline. If you don't have money for the party then you'll have to cancel it.
The consequence for doing this is that she has to give the money back and if she's not able to do that to do chores for you to pay you back. I suggest that it's important to spend time talking with and listening to her. Ask her why she took it, what ownership means to her, etc. Spend as much time listening as talking. Get an understanding for why this happened. Talk with her about how to prevent it happening again. I would also be sure to keep money out of her reach. Treat this as an opportunity to teach instead of the need to punish.
I suggest the ADHD is unrelated to the issue. Impulse control is related. All 8 yo have some difficulty with impulse control. Does she do other serious things impulsively when it's important to maintain control. How are you helping her to manage her impulses?
She stole the first time at her birthday time. Is this theft also at birthday time? If so, that may give you a clue as to why she did it.