So here is my new dilemma

Some one accidentally gave me to much money. Now I have been waiting for them to tell me that they mad an error and take it back. This has not happened. Now this money is now burning a whole in my pocket. I can see all kinda things that need to be purchased with these funds . So am I gonna tell the slap dick that they made a mistake and give them back there money or should I spend it and worry about the rest later.
What would you do?
And yes Fred I already know what you would do