Family Dollar complaint: Fake bills passed off! I received

I broke a hundred dollar bill the only bill I had left for the week.my changed was 70 and some cents I got one twenty one ten and the rest in fives. I found out an hour later the twenty was fake. I took it back and was told they double checked there was two people at the register one being the manager. Told me theres no way of proving it and to take it to the bank. I am positive if you check the cameras you will see proof of when they received the bill and when they pass it off intentionally.two people at the register and the employee was hesitating as she kept look at the manager and he gave the okay then said I checked the bill. The cameras will show he never did such thing and I feel he knew the whole time because he remember exactly who I was and remembered exact change giving claiming it was only one twenty the had in the register at that time.