Design idea

So I had an idea the other day. It's an idea for a game mechanism and it goes like this.

1. The GM sets a difficulty number (challenge ranking, whatever...) from 1 to 5.
2. The player makes a pool of dice which is based on the relevant stat and niche applications.
3. The player rolls the dice, counting 4-6 as successes. For each success they get a plot point to their overall pool (it might already have points in it).
4. The player spends as many points as they wish to tackle the challenge, and keep the rest. At least 1 point must be spent. Spend the challenge number to pass the challenge. Spend more than the challenge number to succeed wildly.
5. Points can be used later to improve the character.

So, an example.

My character, Gus, is going to sneak past a guard.
1. The GM sets a difficulty of 3.
2. Gus has a Sneaky stat of 2, with a bonus around military installations, taking it up to 3 dice.
3. I roll and get 2, 4, 4; adding 2 points to my character's tally of 5 (now up to 7).
4. I could pay 3 points and get Gus past this guard, but I decide to pay 4 points so that Gus gets past the guard and the guard is about to start a really long patrol circuit so Gus has more time for the next actions.
5. I'm left with 3 points that I can either save for character improvement or for the next challenge.

That's the raw version of it. I think there's a bit of polish I could make to it (spending points to help other characters, refining the rate of spend and earning, and so on).

I can't help feeling that it's been done before. Have you seen a system like this? Which game was it and how did it work in that game?