How to determine impact of a new feature that wasn’t an AB test

Hi, I’ve run into a situation at my company and was wondering what all the smart people on here thought would be a good approach.

My company released a new feature for an app that was awarded on a first-come-first-serve basis (the user had to pay 5000 of the in-app currency to unlock it). We allowed only 1,000 users to “purchase” this feature and it sold out in 2 days (there are roughly 80,000 active users a week).

My job …

ab datascience feature impact test