I'm not sure if you have already sent this guest a special offer but from the title I'm presuming you have so read the link below on how to either withdraw or amend a special offer:
Hi @Naomi583 , did you get a chance to explore the article shared by Joelle?
Were you able to amend the special offer by following the steps in the article? Do keep us posted if you need further support with your question! We'd love to support you 🌻