How do we dispute a purchase before it posts?

You can't cancel a purchase, you have to request a refund.

You do not say where you see this purchase. If you see it in an email or a message, and if you can not see it using the methods discussed in the link(s) below, there is the possibility you are being scammed. Click on this link to read how to "Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams" --> https://support.apple.com/HT204759

Click on this link to read what to do "If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement" --> https://support.apple.com/HT201382 Note, if you are a Family Sharing organizer, purchases or subscriptions made by another family member may be charged to your card but will not appear under the your purchase history or subscriptions. Ask family members about those or check your receipts.

To "Request a refund for an App Store or iTunes Store purchase" read this link --> https://support.apple.com/HT204084

Report A Problem to "Sign In for help with iTunes purchases made in the last 90 days" or ask for a refund; click here --> http://reportaproblem.apple.com

For subscriptions, "The Report a Problem website shows most subscriptions, except those for Apple services and iCloud storage charges." [https://support.apple.com/HT201382]. You need to first cancel renewal on a subscription:

View, change, or cancel your subscriptions --> https://support.apple.com/HT202039

If you have unsatisfactory results with Report A Problem or you want to try to request a refund for a subscription, contact:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions --> https://support.apple.com/en-euro/itunes-contact

or

Worldwide iTunes support --> https://support.apple.com/country-selector/itunes