The ethics of finding cash on the street

You are out walking one afternoon when you notice an envelope on the ground which appears to have something in it. Suspicious, you pick it up. It has no name on it and it's not sealed. When you open the flap you are surprised to see dozens of fifty dollar notes.

No-one around you seems to have noticed you pick up the envelope. You have no way of knowing who the money belonged to or what it was intended for.

There is a police station nearby where you could leave the envelope. However, you have two young children and are currently struggling financially. You have recently been worried that you cannot afford essential repairs on your car which you rely upon to get to work. The money you have found would make a big difference to you.