Integration with PayPal, what to do in case of invalid data?

Question

PayPal

I'm integrating with PayPal IPN .
The scheme is simple. We submit the form, the client pays, paypal pulls the pen on our site. Then we pull the paypal to see if it really was a request from him.
It also says that you should check the amount of the order and so on.
Here are just a negative scenario is not described anywhere. So I checked the order amount and it is not correct, or the order number is not correct. What to do in this case?
Those. simple example. Someone makes an order and in the web inspector changes the order amount from 5000 to 5. As I understand it, Paypal simply takes and credits the money. And what to do with such a payment?