Accepting a scholarship for masters just to get the money and fund a future masters
First, my advice is to never think of a graduate school stipend as a way to earn a living (or to save for future goals). Stipends are design to barely cover minimum costs of living, and often fall short of that goal.

Second, I'm not sure whether a Master's degree in theoretical physics will help or hurt an application to a maths Ph.D. It seems that it could be related, depending on the subfield of maths that interests you. 

Third, in many fields you can apply to Ph.D. programs directly without a completed M.S. degree first. Maybe that is also possible in your case.