It's good that you changed your mind and will do the right thing in the future. Theres just one thing I don't get...
would you give the $100 back??? in my opinion i wouldnt, if they made the mistake then its really there fault not mine, but it is wrong to take more money then you should
But.... huh? Isn't that like a total contradiction in terms?
I'm glad you saw your mistake though, and will correct it next time.