Keep in mind that SAP doesn't work like some other applications with events on every user action. What user exits are triggered depends not just on the buttons pressed but on where they were pressed and what data was entered. So the requirement to validate "on Enter" might not always be feasible in.
But why do you need a partner validation at all? The best solution would be to have the partners determined automatically. The users should get to choose / enter only those where automated entry is not possible and a human must make the right decision.