How do I correct a migration error? i.e. an inactive employee's payroll migrated incorrectly to the tune of $25K .

Hello there, @office-saintjohn.

Allow me to share some information and assist you so we can migrate the corrected information from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO).

Yes, I agree that we'll have to edit the transactions in QBDT since the employee is inactive and inactive employees can't be migrated from QBDT to QBO. 

Referring to your next question, we're currently unable to migrate transactions only. If your QBO company is less than 60 days old, I'd recommend deleting your entire data and starting again. No worries since it won't let you pay for another subscription again. Before doing this, make sure to check the following:

• A purge of your business may permanently harm any ties you have to banks or other third-party apps.
• This process doesn't cancel your QuickBooks or Payroll subscriptions, it'll just clear out your book. 
• This is only applicable in QuickBooks Essentials, Plus, and Advanced.

When ready, please follow these steps in deleting your company data:

1. Sign in to your QBO account as a company admin.
2. Go to the URL in your browser's address bar. Delete all the text and enter the following web address instead: https://app.qbo.intuit.com/app/purgecompany
3. To confirm you want to delete your company data, enter Yes.
4. Select OK, then Wipe data.

Once finished, you can now start your books from scratch:

1. Sign up for QuickBooks Online.
2. Enter your user ID and password. You can use the same user ID and password as before.

You can now then perform the migration process once again with the corrected information.

You can click on this article to know how your books were moved from QBDT to QBO since your books may appear differently after: Learn how features and data move from QuickBooks Desktop to QuickBooks Online.

If I can be of any additional assistance with regard to managing your data, please don't hesitate to insert a comment below. Stay safe.