I am asking you to imagine a situation in which oil is priced in currency A but the proceeds are stored in currency B in order to determine whether it is A or B that matters, or both.

In your example, A and B are the same (dollars) and you demonstrate that there is a need for the common currency of A and B, but you are unable to distinguish between the demand caused by A and the demand caused by B, so I don't think you are groking the example. Suppose A and B were different. Oil is priced in dollars but the proceeds are stored in yen. So

   1. France sells euros to buy dollars
   2. France gives the dollars to KSA for oil
   3. KSA sells the exact same number of dollars for Yen.

By the way, this used to be an adage of currency traders - it doesn't matter what currency a thing is priced in, what matters is the currency where you store the proceeds of the sale. This was shorthand for "the seller determines which currency gains from a trade, and the buyer determines which currency loses". So France, in selling euros for dollars, determines that euros will lose value, but does not determine that dollars will increase. It is Saudi Arabia, the seller, in choosing where to store the proceeds that determines which currency, in this case, the yen, will increase.