Medicare cannot negotiate drug prices even with the power they have now. If we are offering this Medicare to all, then that same Medicare doesn't have price negotiation.
Unless that is another in the long list of things that are part of Medicare that isn't going to be part of Medicare. Medicare 4 All is the ship of Theseus of Medicare.
Presumably any legislation that changes Medicare to that extent could also change the prohibition on the government setting the prices it pays for medication. The current status of non-negotiable drug prices for medicare was controversial from day one - as it should be because it is nothing but a blind giveaway. Medicare is not set in stone, it never was and can be changed through legislation. Nearly every federal program deals with at least some level of legislated change every year via the appropriations bills, and frequently other bills as well.
Legislation is legislation. We used legislation to set up the current Medicare prescription drug plan, we can use legislation to change it.