Family Dollar

I went to this store around 4:20 on 8/16... We bought some tp a birthday card and a dollar gift... My total was $12 and some change, I handed the cashier 2 $40's so that I could break one into 2 $10's and she puts them into her register... She proceeds to give me the change from my purchase and told me I had to give her the $20 to break it. I told her I did and she said no... well we asked the manager on duty and he said the he would have to call his boss because he didn't know how.. I told him I had to run my son into town and hopefully by the time I get back they will have had a chance to look at the footage...
Nearly 45 min later I called on my way and he HUNG UP ON ME! After that come back and the manager on duty asked me what I said to her because she quit... I don't think it ever occurred to him that she may have quit because she stole... The only thing I said was that I spent $120 earlier in the day and I know I had $80 from the change... I'm the type to give the cashier back money if they miscount, I guess not everybody thinks that way...
At this point (I have it on video) he tells me that if I want my $20 I'm going to have to sue them and to get the "[censored]' out of his store!