I'm thinking of pretending I don't know the "diamond" is fake to avoid hurting my profit.
Why would you lie about the authenticity of a diamond?
Remember what I said to you yesterday.
It's extremely unethical to lie about a product. You should be honest with the type of product that you have. Continuing the lie about your products will result in people not trusting you down the line.
It's okay since I won't need to sell it anymore. I sold it to another guy.
You should tell him the truth about what you sold him. He needs to know that it is not really a diamond so he does not unintentionally scam someone else.