What to do if the grocery asks you to pay penalty for item you forgot to pay for?

I drank a soft drink of 3 dirhams from a grocery shop and forgot to pay at that time. And after going 5 times and shopping with them, I had also forgotten that.

Now, while shopping the sixth time, they informed me that it was in their CCTV footage so I said ok, take it now but instead of that, they forced me to pay a penalty of 500 dirhams.