I purchased a dog for 100 dollars. The lady later said all I gave her as a 20 dollar bill, which is dumb because why would she let me walk out if she was 80 dollars short. She kept telling me go back and give her the rest when I said no I already paid you she said she is going to call the cops and press charged. Unfortunately there as no receipt or proof of purchase. Can she press charges?

Sure, she can TRY. You have little defense, no receipt and paid cash! Her word against hers. Any other witness? Does she know you and where you live?

Of course she can, again, it is not really theft and I don't see a criminal case, She did give you the dog, even if she expected payments,

So she would have more of a case for civil court.

No witnesses the only information of mine she has is my phone number so I assume police or whoever can find my name that way. But if I have no proof neither does she I am pretty sure she just trying to pull a scam but in your opinion would a judge really be able to make a decision with just her word against mine.

I don't think this is a case a judge would hear, not for $80. Maybe small claims court. Police are given some discretion in this type of case and they, or the court, may opt to do nothing.