"Federal government is a “USER” of currency(Federal Reserve Notes) like everybody else. Fundamental principle of MMT is false."

No other entity besides the federal government can "create" US dollars when it spends. Banks can create credit, but that must be retired with dollars created by the government. You admit to this when you stated that retiring a debt decreases the total money supply. A constantly growing GDP can mask this by "rolling over" private sector debt into new debt, but that fails regularly when the economy slows or Congress foolishly decides to balance its budget in earnest.

The federal government "uses" the dollars it creates, so you are somewhat correct in that context. However, it never has to first obtain those dollars from anyone. It simply decrees them into existence in the private sector. It is entirely unique in that regard. It can't borrow what doesn't yet exist, so your assertion that the Fed funds the Treasury is incorrect.