I had the following situation a few weeks ago.
I took some fish back to my lfs and they gave me some credit. At the same visit, my kids wanted some more fish, so we chose a few. The person who helped me wrote the price of the fish on the bag - the net amount - total of fish less the credit they had given me.
I then went to the till, and the lady there knew that I had brought some fish in and gotten a credit, so she nocked off the credit again.
So there I was, thinking to myself, what do I do? Do I keep quiet and take the fish with the extra credit? Or do I own up? Had a few seconds to decide?
I ended up saying to the lady that I thought the credit had already been applied to the cost of the fish.
Thought to myself, that I go to the fish shop often. The 2 staff members might chat about it later and realise what had happened. And it was only about £10, so I thought not worth having a problem there at a later stage.
If it would have been a lfs that I hardly ever go to, that might have been a different story....