I think it's tricky to generate made-up games and rules to teach a point, especially when you are conflating two life lessons into the same thing (here, the concept/value of money/importance of saving + the concept of delayed gratification). I would be more inclined to let "money and things" be "money and things" -- i.e., if he wants a toy that costs $X, then he is going to need to save up for it, through allowance or extra chores or whatever. Note that this naturally teaches delayed gratification anyway, as it will take some time to save up the money; you don't have to invent some special lesson to teach that.
You can also teach delayed gratification on its own through your behavior, as appropriate for his age/abilities -- e.g., if he calls you when you are wrapped up in something, you tell him that you are busy but will be with him as soon as you are done (and that may be 10 seconds at 4 vs. 5 minutes when he gets older); he wants a snack but dinner is in 15 minutes so he can wait (or he wants dessert first but has to finish his dinner first); etc. Over time, that daily reinforcement of the concept is going to have far more impact than a made-up game.
The problem with made-up games that are sort of divorced from reality is you never know what lesson they are going to take from it. You think you are teaching him delayed gratification -- but maybe he thinks the lesson is that when he does XYZ, mommy buys him toys. I think kids are more likely to get the "right" lesson when you engineer less and keep the causes and consequences as close to the way things naturally work as possible -- it's just much more natural to say, hey, you want a toy, ok, toys cost money, so let's think of how you can earn that money so you can buy it.
Tl;dr: "Teachable moments" are such because of the *moment*, not because you figured out a special way to force the *teaching*.
