This post is about something I don't understand.
Let's start out simple. There are two parallel imaginary worlds: the green world and the red world.
In the green world people use positively-valued green money as the medium of exchange. If I buy something I give the seller my green money in exchange. Green money flows in the opposite direction from all other goods and financial assets. I am not allowed to take green money from someone without their consent. Only the issuer of green money is allowed to create green money. The borrower of green money undertakes an obligation to give green money to the lender at some future date.
In the red world people use negatively-valued red money as the medium of exchange. If I buy something the seller gives me his red money in exchange. Red money flows in the same direction as goods and financial assets. I am not allowed to give red money to someone without their consent. Only the issuer of red money is allowed to destroy red money. The lender of red money undertakes an obligation to accept red money from the borrower at some future date.
There is a symmetry between the red and green worlds; one is the negative mirror-image of the other. But there is also one asymmetry: the red world has a fundamental problem. Each individual can increase his utility by buying more goods and selling less goods, thereby accumulating an infinitely large stock of red money. The bank that issues the red money needs to put some limit on each customer's holdings of red money, to ensure this does not happen. This is not a problem in the green world, because having zero stock of green money sets a natural limit that stops an individual buying, and the individual is fully aware of that limit.
The real world is a red/green world. It has both green money and red money. A positive balance in a chequing account is green money. A negative balance in a chequing account is red money. If I sell my car to Andy, who gives me a cheque for $1,000, the bank reduces my overdraft by $1,000 and increases Andy's overdraft by $1,000. The bank has transferred $1,000 of red money from me to Andy. IIRC my father nearly always used red money. He nearly always ran an overdraft, paying it off once a year when he sold the harvest, to keep the bank manager happy.