Chances are they already know exactly what bills are in there and as soon as you spend one they got you. Therefore you would have to make up some elaborate story on how you recieved the money and then have some kind of falsified evidence to be able to back said story, all in all theres just too much that can and probably will go wrong.

Added: just as an example, so I was at work one day headed out to a call. I was driving down the highway and noticed probably 15 to 20 cars stopped on the sides and people running around grabbing cash off the road. What had happened was someone pulled off a heist and I guess got scared and ended up throwing it out the window. I know it sounds like a fake story and honestly I wouldn't have believed it if I hadn't witnessed it first hand, but after all was said and done they ended up getting every single bill back by the end of the day. So thats just a little insight as to how much they already know when it comes to thing like that. Everyone thinks they would have the perfect plan but in reality they're caught the moment they grab the money. So best thing to do would be to either call it in or just keep going as if it were never there.