The basic concept is that you cannot owe a debt to yourself. The Fed cannot owe a debt to itself. To the Fed, the printed but unissued Federal Reserve note represents nothing more than what you would have if you typed up a promissory note at home that said "Firrtree hereby promises to pay the holder of this note the sum of twenty dollars on demand." As long as you, Firrtree, hold that note in your possession, the note is meaningless. It has no legal or economic status. But if someone cuts your lawn and for 20 dollars, and you say to that person "hey, will you let me owe you until next week and just take this promissory note as evidence of my debt to you?" that note becomes evidence of a debt you owe to that person AT THE MOMENT YOU ISSUE THAT NOTE to that person. 

 As long as the note is in YOUR possession, it is neither money you own nor a liability you owe. 

 With the Fed, it's the same thing. The Federal Reserve note, when held by the Fed prior to its issuance, is neither "money" nor "money owned by the Fed" nor a "liability owed by the Fed." At the moment the Fed ISSUES that note to someone (usually, to member bank like Wells Fargo or Bank of America, it becomes a LIABILITY owed by the Fed, not money OWNED by the Fed.