First, as has already been mentioned by others, grants generally do not allow using funds freely. Funds can generally be used only in accordance with the plan submitted with the grant application.
However, leaving that issue aside for a moment and assuming the advisor can use their grant money as they please, let's focus on a different aspect: what's in it for them? Is there any benefit for your advisor in hiring you as a research assistant for one year before you start a PhD program?
If there's none, then you're essentially asking your advisor to take on additional costs because of your desire to obtain PR. Researchers are significantly more expensive than PhD students.
You may only have leverage if you are highly valuable to your advisor and are prepared to leave if they do not fulfill your request. However, MS graduates are typically not considered highly valuable, or at least not seen that way by professors. Your advisor will likely find another PhD student if you leave.
Also, keep in mind that a negotiation means either party can say no and walk away. Can you say no? Are you prepared to leave your advisor? Do you have any other options in Canada? If not, and your advisor knows this, then it’s not really a negotiation.
It might also be unclear to your advisor why you need PR now if you plan to stay for a few years as a PhD student with them anyway. You don't need PR to do a PhD with them, and you could get PR after getting your PhD. Your desire to get PR now might be seen as a sign that you aren't really planning to spend a few years to complete your PhD.
Given the above considerations and the fact that your advisor has already indicated their unwillingness to agree, I'm afraid that your chances of getting what you want may be non-existent and that any attempts to push your advisor can only do harm to your relationship with them.