Tried making a market code, just a beginner 2, 3 weeks learning python

I tried building a code for a internet market store, but the problem is that when I let's say have 30 euros and the product is 50 euros, when I try to buy it it says that I can't buy it, and my new account balance is 0 euros. I want to make it to stay that way - 30 euros. I'm writing this so you can help me by typing some examples for this kind of stuff. Thanks in advance!