Wow, Putin is 'annexing' Crimea, Iran is trying it's best to build a nuclear bomb to annihilate it's neighbors, ISIS is beheading Christians, Jews, Journalist, and anybody else it deems 'unworthy', Sexism and Racism plague practically every society on earth, disease and hunger kill off thousands of children and adults every day, and the list goes on......so I hope HE isn't wasting a lot of time on your little white lie.
Like it or not, the 'damage' was done the second you decided to lie and take the extra 50 bucks. What you're experiencing now is nothing more than guilt and putting it out there as a "What would YOU do" post is just a way of trying to deal with it.
If she asked me face-to-face, I'd tell her $150. If she (without asking) sent me a check for 200.00 or 250.00, I would pocket it without a second thought and chalk it up to her incompetence.
The die has been cast, the lie has already been told. Anything you do now is for YOU and to ease YOUR conscience. It's the moral equivalent of cheating on your wife and begging forgiveness later.
In the end, do what YOU think you'll be comfortable living with....and do it for THAT reason, not because someone or something is 'watching you'. Hey, I assumed you wanted an honest opinion, not some sugar-coated warm fuzzy.
chas