stick-up has two meanings :
One meaning is hold-up,that is,a situation that stops something from happening or making progress. traffic hold-ups etc..
The other meaning is an attempt to rob a person or place by threatening them with a weapon.
I am unable to draw parallels between your example sentence and the actual meaning of the word. My assumption is that the girls haven't attempted extortion. They might have cajoled the cashier into exchanging the torn notes. Because it had made them richer, therfore, they have got more than their share. Sorry if iam wrong.