As I understand it, after you've been hired, the person who hired you wants to change your contract in a way that would be more advantageous for them. Well, they can ask, but...the whole point of a contract is so that you (and your employer) can be held to the parameters you agreed upon.
I must say that although I've been in academia for my entire adult life, I still find myself baffled by some people's academic cultural expectations. You are considering taking on what you view as 140 hours of additional work in the remainder of the calendar year. That's almost a month of full-time work. It is absolutely absurd to me to do that amount (or anywhere near that amount) of work "as a favour." Are you not a highly trained professional? Don't highly trained professionals (in particular!) get paid for their work? Again, even as a lifelong academic, I've always thought so.
On the other hand, most academics I know are at a lot less than 100% research. It sounds like you currently have a temporary, "soft money" academic job. Do you want a permanent academic job? If so, that will probably involve some teaching. (Right? This is probably field dependent.) In that case, getting teaching experience is not just a CV-stuffer: it is actually valuable for your future. Getting used to a balance of research and teaching is of some value, at least in my neck of the academic woods, where most tenure track faculty eventually teach at least 1-2 courses per semester while maintaining their research activities.
But back on the first hand: if you are currently at 100% research and are happy with that, then I understand how you are viewing this as time away from your research and are not thrilled. If that's how you feel, then that's how you feel, and I think you should just say no, explaining that you are really excited about doing what you are doing and would like to continue with it. You gave your PI some chances to come up with something in exchange, so it's not as though you will come off as completely unwilling to help. Being unwilling to take over someone else's work in exchange for nothing is really not a character flaw.
Finally, I don't quite understand the financial aspects of the situation. Why would you teaching the class generate funds that are not generated by the PI teaching the class? Maybe I'm missing something here.