Of course, the alternative is what is known as "Helicopter Money", when the central bank directly purchases bonds issued by the Treasury and forms the backbone of the MMT monetary cult.
But what if there is at a several day interval between Treasury issuance and subsequent purchase? Well, that's perfectly legal, and it's something the Fed has done not only during QE1, QE2 and QE3, but is continuing to do now as part of its "QE4/NOT QE."
On December 16, the US Treasury sold $36 billion in T-Bills with a 182-day term, maturing on June 18, 2020, with CUSIP SV2. And, as shown in the Treasury Direct snapshot below, of the total $34.3 billion in competitive purchases, Dealers acquired $23.7 billion.