Well done mate, A similar thing happened to me a few years back, A similar amount of money too.
In my case the wallet had a photo membership card for a caravan club. The guy was of Arabic appearance, I still remember his name, Sawiki. I sat in the car counting out the money. It was just before Christmas, and I have to be honest I was tempted. At the end of the day I knew the money wasn't mine so went to the police station with it. An hour later this guy posted a thank you note through my door with a £20 note.
It's the same when a customer tells you that they are going away for 3 months, but still want you to do the windows, you just have to do what's right.
That hug from the old dear will be your best Christmas present for years to come mate, you lost nothing, it wasn't yours, but you gained one hell of a lot. What you were able to do for her, neither of you will ever forget. Dai