Conflict of Interest in the US Government

I worked for the U.S. Marine Corp as a civilian. I also worked for a company that paid me a commission. The USMC terminated my employment saying that it was a conflict of interest to receive a commission from a company doing business with the US Goverment and being an employee of the USG at the same time. However, the company that paid the commission to me was still able to do business with the USMC.

Is there a law/code that would bar that company from doing business with the USMC since they were the payer and I was the payee?

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