Basically, you contributing / creating OSS has to be tangential to what you charge money for. IE, your code is critical to your service or business, but not what you're selling.
For example: My employer might open-source a C# adapter for an open-source C library. We won't make money off of the code, but it's a way to promote ourselves within people in our industry.
I once met a guy who consulted on making realtime web applications in 2010. (Before websockets.) His library and protocol were open-source; but the service was making changes to real commercial web applications.