Is any way to create subscription/agreement for paying in any time?
For example: user once sign agreement for x-site.xx and system will be create invoice for different price in any time, but user will not need sign again.
May I read about it in documentation?
something like that implemented in Myworks Paypal gateway for whmcs
I suggest you to try out Billing Agreement - Reference Transaction.
Read it here https://developer.paypal.com/docs/classic/express-checkout/integration-guide/ECReferenceTxns/
You can have an agreement with the buyer and charge them anytime you want with any amount you wanted too.