poor enxperience

I had a poor enperience with fooding shopping in a korena store today. I paid the casher $30 for the total amount $26.11-which is supposed to be paid, the casher took the dollar and put them to the drawer and making the changes to me, I told her that I have 11 cents and gave them to her, she put it in the drawer and she got confused, she can't remember if I gave her 20 or 30, because in her compter she put two payment, one is $20, one is $20.11, but she knew I only pay her once, I told her I paid $30 and then 11 cents, she looks like doesn't know how to deal with this, then she called the manger, and the manager assumed that I gave the casher $20 not the $30, then the manager held us in the store for more than 2 hours and I have keep on telling him that my son is hungary and I need to resolve it quickly, he told us to wait until the cash finish the whole transition, then he can count the money, finally he count the money, then he said he is in short of 8 dollors, then he got decision that I ahve only paid the casher $20 not $30, otherwise they would have have more money instead of in short of money, I totally disagreed with this, then he called the police, the police came and doesn't help, the police want us to sue this to the court, after I got home, I realized that I have been helding in the store for very long time and my son got stomache, I felt very awful due to this, I am thinking of finding a lawer, but I don't know if there any lawer would like to take this kind of case. Can anybody give me help?
I am very appreciate!