So you basically want to cheat the user. Slot games are inherently random. Yes, you set a possibility of winning, but that's it. You don't decide if the user wins or loses.

The best way is to generate a random combination, then calculate the winning on that (2 of a kind, 3, royal flush). If you want to let low-earning users win a bit more than usual, just lower the number of permutations.

An example:

User has 2000$ and rolls once: He is given 5 x 5 x 5 slots. However, if the user drops under 1000$, he's given 3 x 3 x 3 slots. (This dramatically increases the chances of winning, but the animation looks the same, so the user doesn't realize what's happening.)