I don't follow the example. In response to the bold, wasn't that the case either way?
No. If you held stock that was still worth $20, it is something that the cash in circulation must be able to purchase.
Ok, try to visualize all of the value in the nation (stocks, houses, cars, labor, consumer goods, artwork, and frozen concentrated orange juice). Now try to visualize all of the US currency in the nation (this is not stock, not houses, not cars, not frozen concentrated orange juice, just US dollars). The value of the dollar is, to first order, the number of dollars in circulation divided into the total amount of value it has to purchase. Now, that's a super lazy way to calculate the value of the dollar, because some of it is effectively not in circulation, and some of that stuff is not for sale, etc., but this is a thought experiment, not a science experiment.
So if your stock goes down in value, the the total value of all of the things that dollars need to buy goes down. That makes the value of the dollar go down. Because the number of dollars in circulation stayed the same, and the value it has to buy dropped. So when the stock market crashes, or housing markets crash, or the economy shrinks, there are more dollars as compared to things you might trade dollars for. Therefore, the value allocated to each dollar shrinks. Sometimes it works the other way, some other factor forces the value of the dollar to rise or fall, and commodities rise or fall in response to the rising and falling value of the dollar. So for example, pretend that there is no market crash, and value remains constant. But suddenly there is a huge influx of dollars. Now again there are more dollars to buy the same amount of value. Therefore, the value of the dollar shrinks. This translates to needing more dollars to buy something.
If stocks are overvalued, meaning they are simply not worth as much as we pretend, it means that the value of the dollar is artificially high
in at least that respect (because the value of stuff it has to buy is artificially high). So if stock value evaporates, the value of the dollar is pushed
down (inflation) not
up (deflation).
Now if stock value evaporates for a different reason, like say, people wanting to hold a more stable investment (like dollars) instead of stock, as happened earlier this year, it puts a big demand on dollars relative to stock. If people buy and hold dollars, instead of exchanging them, the value increases. We've seen moves toward holding dollars this year because of the pandemic and because of social unrest. If we return to anything resembling stability, we should see people relax and release those dollars (this is already happening), which counteracts deflation. When that happens, we'll notice that more dollars are in circulation than used to be, which further counteracts deflation.
Deflation and inflation of currency should not be confused with deflation or inflation of other commodities or goods. Lots of things have their own independent inflation or deflationary pressure on prices. And it's not the same as deflating or inflating currency. Consumer electronics, for example, perpetually deflate.