I am using the Williamson Macroeconomics textbook to look over a chapter on Monetary Policy and Banking (Chapter 12, 5th Edition).
For a bit of context, the author specifices the government budget constraint as:
$$PG+\left(1+R^{-}\right)B^{-}=PT+B+M-M^{-}$$
wherein it can be understood as a flow budget constraint. The term $PG$ is the amount in dollars of government purchases, and the term $(1+R^{-})B^{-}$ is the payment due on outstanding government debt. The LHS therefore corresponds to government outlays. The RHS consists in nominal terms, government recepits. The first term $PT$ measures nominal taxes, $B$ denotes government bonds, and the final term $M-M^{-}$ deenotes change in nominal money supply.
The question relates to what would happen the nominal money supply increased (the last term $M-M^{-}$ increased).
The explanation on page 455 is as follows: because both sides have to be equal, and one part of the RHS has increased, something has to give way. That means either $PT$ has decreased or $B$ has decreased or both to preserve equality. In the case that $PT$ does not change, the author explains that the government could reduce the quantity of bonds $B$ that it issues during the current period. This is an open market operation wherein the fiscal authority issues debt, and the monetary authority purchases some of this debt by issuing new money. In essence, the author argues that $M-M^{-}$ increases as $B$ decreases. This does not make sense to me, however. If the fiscal authority were to issue more debt, would $B$ not increase? That is, the quantity of bonds in the market would go up in this open market purchase. Am I missing something obvious?
Many thanks!