What happens if I keep cash sent to me instead of keeping a portion of it and returning the rest to an address in the United States? They have my address but no other information that I can recall. They did not ask me to send them a card or deposit into my account. I was told to keep $100.00 and return the rest of the cash to them.
It sounds like you were sent cash? In the mail? If that's the case, I would strongly advise you either return the money in full and insist you never be sent funds again, or call the police. I would opt for option 1.
This money is surely coming from organized crime, you are being used as a "money mule", or as a vector for money laundering. These could be very dangerous people, and they have your address.
Here's why I would return the money in full and tell the people never to contact me again:
In my old neighborhood there was a guy who I used to see around, fairly young and chatty always smiling, the kind of guy who hangs around and talks to anyone who will listen. One day we were waiting at a traffic light, and he told me "I woke up this morning and found out I was rich! I went to the bank machine this morning and I couldn't believe it! This guy I met in jail got me a job, just like that, I'm rich!"
A couple weeks later I saw him again, he wasn't so cheerful, both his arms were in casts from his shoulders to his hands.
Don't take thousands of dollars from organized criminals.