We talked a lot yesterday. From my observation, he is asking for an immediate salary raise if he starts PM work. But the boss won't allow that. He has to prove his ability in the area before getting salary raised.
There are several issues here,
1) PMs' salary is high than others, so even senior DEVs want to be a PM to get more money no matter he loves PM work or not;
2) There's no work-flow defined for internal work rearrangement.
3) It's a PM-Most-Important principle there, so other work seems to be not so improtant since not properly paid.