Etiquette?

So, what is the proper etiquette when you find money laying in the street? Today I ran errands with the girls and one of the stops was to Target to print out pictures for a present I am making. After we left the store, I was taking the girls to the car and putting the cart back. After I got Hayley out and was walking back to the car, I saw a nice little wad of money laying on the ground. I picked it up, not even looking at how much and starting scanning the area. I stood there for about 5 minutes looking around, I am sure I looked like a complete weirdo to everyone who saw me and didn't lose the money. But the money I saw on top was a $20 and there was more inside so I figured someone would be missing it. Someone who went to their car and then looked for money. Or went in the store and went to pay but had no cash. I thought or a second that I could go in the store and give it to someone who worked there but does that really work with cash? So, after the 5 minute standoff between me, myself and I....I left. I felt so guilty, like I had just stolen from someone. When I got in the car, I looked and it was $40!!!!!! I was so scared to have it, again I felt like a thief. I picture someone crying searching frantically for their money. So, I decided I was going to hold onto it and give it someone who needed it. Someone I see on the side of the road or something, I will know when I see them. But I still feel guilty!!! What is the proper etiquette?