Is proving a point worth this?

"Grocery store had this sign up that said if an item rang up higher than an advertised price it was free. It was the '80s, and stores did stupid like this that I never see in stores today. Mom was buying a box of Little Debbie cakes and they rang up for $2.85 instead of the advertised $2.50. So now mom wants her free cakes. Cashier doesn't know what to do, summons a manager. Manager tells her to ring up the sale otherwise and he'll be right back. Comes back and hands my mother 35 cents cheerfully and says 'There you go!' My mother points out the sign behind him and he says, 'Oh, the last manager put that up, it doesn't make any sense. I'm the new manager and I just haven't had the sign removed yet.' Mom insists they honor their sign, he says nah. Now, up to this point, I as an adult looking back am totally on board with mom's actions. Then, well, things went a little sideways."