I am making a module for the sale of credits for an online reservation site with memberships. I already have functionalities so that if the client buys from the website, the credits are assigned to the user (partner) who makes the purchase. Then I would like to be able to sell those same credits from the POS, but to be able to carry out the sale I need again to select a user (customer) to whom the purchased credits are going to be added.
I have tried to send a ValidationError and UserError from the `action_pos_order_paid` method but this is only shown in the console and does not stop the purchase process. 

What is the method to inherit and interrupt the purchase, or how could I send a message to the frontend of the POS so that it does not allow validation if it does not have any selected user?