If money not yours and you willfully took it knowing there was an error than may be criminal repercussions here if you get caught.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
No, you can't keep it.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
No. You may not retain this $. Aside from the legal issue, consider the ethical dilemma: say YOU accidentally over-paid for a service or merchandise, you would expect the over-payment to be returned, correct. Do the legal, and the right thing and notify the MasterCard bank and return it.
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