@bone
Just typing out loud so I understand...
Your use of the word "buy" makes me think these T-Bill purchases are not overnight Repo loans.
These short-term T-BILLS are taken out of the market, and assuming held to maturity, the face value of these short-term bills gets extinguished... no interest need be paid, whether coupon, excess reserves, or otherwise. This debt extinguishing offsets NEW issuance of Congress approved debt (and/or other methods of money creation). If I am not mistaken, THIS IS QE!
If the buy is more conventional, where the bills are used as collateral for a loan, then the borrower will pay interest, which also offsets fresh money creation based on difference between coupon and loan rate. This is not QE, but rather a longer term repo.
Am I missing something?