You are involved with someone who is trying to scam you. The way it works is that they provide you with what appears to be a real authentic check, when, in fact, it is a fake check. But, your bank does not know that and cashes it, giving you the money. Whether you keep all of the money or give part of it back, the check will bounce and is not worth the ink used to print it. You will be left owing your bank the amount of the check and any bounced check fees. By the time the check is found to be fraudulent, you by then, would have given the money to the person, kept some yourself and in the end, owe it all back to your bank.
There are also legal ramifications you can incur by being involved with money laundering and fraud.
Not good. Do not be stupid and do not accept any check or cash it. Bring it, with the envelope it came in to the police department. If the person who sent you the check bugs you for the money, tell them that your bank refused to cash it.