Control Minecraft money system?

I'm currently writing scripts on a small moneysystem, which should debit taxes with the command / pay. How can you do that?

Here the source:

I don't know anything about script, but you will certainly be able to subtract percentages. Otherwise, you can divide by 100 and then multiply by 81 (because 19-100 = 81).

The question is how, -10% was planned

Then you just have to divide by 100 and then multiply by 90 (because 100-90 = 10)