How to solve the problem with cookies in the affiliate program?

Question

HTTP Cookies

There is an affiliate program.
For partners, a link is provided in the form domain.com?ref=partner_login
When a user accesses this link, we write partner_login in the cookie. But the user does not register on the site, so we cannot bind cookies to some user (since he does not exist yet - he is not registered).
Then, after a while, the same user logs in from the phone directly via the domain and registers.
The problem is that the partner knows this user and asks why we do not charge bonuses to him.
How to get around this problem?

Answer 1

No way, this is the essence of affiliate programs - the user must register from the same device within N time, otherwise we cannot establish that this is the same user. Motivate in non-technical ways - give a gift to a new user if he registered on the recommendation.