Professor expects me to attend a conference. I can't afford to go,even with 50% funding
I am really hesitant to discuss my financial shortcomings with Prof.
  M, or else he assumes that I can afford the travel.


I think you need to get past this. You do not have financial shortcomings, you just don't have money. Working on a PhD without family financial support, with existing student debt, and with limited school support is not easy. I respect you for doing it. Take pride in making the hard choices that involves.

With that attitude, explain to Prof. M that you cannot afford any travel expense, and will only be able to go to the conference if your expenses are fully covered. Check any funding sources he suggested, and ask advisors at your current university if they know of any sources.

From comments: Also check with the conference organizers. They may have scholarships or be able to discount the conference fee, and find free or inexpensive accommodations. You may need to work several angles to align the total cost with the available funding.

If you can't get the full funding you need, tell Prof. M. that you regret you cannot attend the conference. At the worst, you will have to withdraw the paper. However, there are alternatives the conference may permit.


Remote video presentation
If anyone from your current university is attending, ask them to present the paper
Prof M. may be able to get a current student or a collaborator who is attending the conference to present it.