what you need is to make it so your character has stones stored as variables...so like make a tresure chest and have it say gained 3 stones, add control variable stone = 3.
then where I had the shop instead of having the skill 100$ have it 2 stones. then after the hero clicks yes to buy it make a conditional branch:
Variable Stone greater then or equal to: 2 (with else)
- Control variable stone - 2
- Change skill (add the skill he buys)
else:
- Text: you dont have enough!