You will be developing a given power at a given fuel flow rate.
You can run an engine at a given RPM and a given MP, and vary the fuel flow rate (by the mixture control) and the power output will still vary - because you are varying the fuel flow rate.
You are right in that if the RPM is decreased (by increasing the load) then the torque will go up, and the power remains about the same. That's why if one is flying behind a VP prop, and fiddles with the RPM lever, the speed (which is the best measure of power) doesn't vary much at all.