Paypal - how to prohibit paying an invoice?

Question

PayPal

Actually the question is how to forbid paying the bill if, for example, the user has forged a page, or, well, or some parameter does not suit me?
With webmoney, the situation is as follows: a user on my site fills out a form and sends it to the webmoney site. Those, in turn, send me $_POST in response, which contains information about the payment. I check it and if it suits me, I send YES in response. Next, I receive a response from webmoney in which the final information about the payment.
And Paypal immediately receives IPN information about the payment that has already taken place + immediately there is a request to check the status of the payment, after which the final information about the account comes.
I test everything in the sandbox, I took the code from their github

Answer 1

I seem to be starting to understand. The fact is that I use a button from the constructor, and it has pre-set parameters + the ID of the button itself. Now I tried to change the name of the select - and paypal noticed the substitution. That is, it turns out that there is validation and now it only remains to wait for the status $_POST['payment_status'] = 'Completed'
Did I mess up anything?

Answer 2

Read the documentation.
Everything is written there and there is no need to compose.