I'm conflicted on this one because of a very recent incident that happened to me and my fiancé. We were meeting at a local winery with my siblings to celebrate my dads birthday. I had been suffering financially because of a huge pay cut so my fiance footed the bill for our evening out.
Before we arrived she handed me some bills which I stuffed into my pocket. Long story short, I lost the cash then was mortified to learn that she handed me five twenty-dollar bills. Somehow that cash had worked its way out of my pocket and landed on the floor. I remember several guys that were standing very close to my chair as I was sitting. I'm sure one of them used his foot to shuffle the money towards him to pick it up.
In my case it was obvious who the money belonged to, in your case it was not but since you found that cash in a store there is a good chance the person who lost it may have inquired about it at customer service.
In fact, just last week as my fiance was again paying for our weekly groceries at Walmart, I spotted several bills laying on the floor between two registers. I picked them up then asked the customer at that particular register if they were hers and she said no. I gave them to the cashier who in turn gave me a puzzled look like I was crazy.
I'm not trying to give a lesson on civic responsibility because I have found cash and other valuables many times and kept it, but usually after attempting to find the rightful owner.
I can't help but wonder if the money you found belonged to a child who was Christmas shopping.