Course load expectations for teaching track and how to negotiate for courses
As usual, there is no clear answer. People are different. "Requests" are different - maybe, the request in your case was an honest, open-ended question and your department head is not surprised that you said No. Maybe the request was supposed to be a rhetorical question, and the department head was really surprised that you said No.


  How does one manage such a "request" from the department chair?


Well, as you would handle any request. Check if it is ok for you to comply with it (do you have the time? is complying with the request aligned with your interests?), and if it is, agree. If it isn't, carefully explain why you can't do that, and offer alternatives. From your descriptions alone it does not sound like you did anything particularly wrong. However, most people are a bit more accommodating when starting a new job, so I would say that your department head was probably at least a little bit surprised that you turned him down.


  Did I shoot my career really early in the foot by declining such a request 


"Shot in the foot" is maybe a bit strongly formulated, but it is certainly possible that the head has the impression that you are not overly flexible. I mean, asking somebody to teach a second course is not really an unheard-of request.