Schedule 2 separate calls.
The person books a call using the promotion link (or just set your hourly rate to 0), then, change the rate, and have them book another call for a time which is directly after the first one.
Alternatively, let them book a paid 30 min call (at half your usual rate) and then talk for 60 minutes - so they’ll only be paying for 30min I’m actual fact.
Good luck