Once you change the account email, the guy with original email will get an email from Instagram about "email change" tell him to forward you that email . The email is about Instagram asking you whether or not it was you who changed the email, if not you then they ask you to "Secure your account" . so all you have to do is to click on the "Secure your account" option and and it will take you to a page where there will be two options. One would be yes it was me and another secure my account. so just click on yes it was me and you are all set.I suggest you should do your transaction through this Escrow service to minimize risk.
https://viralaccounts.com/viralaccount-escrow-service/