How do I search for purchases where tax was manually entered?

Hi TanyaMc , sounds like a bit of mess! From your post, it sounds like they've changed the tax amount or rate in the Tax information window from the purchases:

If that's the case, what you can do is run the Company Data Auditor (from the Accounts command centre). In the Tax Exception Review section, the Company Data Auditor will check for any tax variances, including those on purchases. If it finds any, you'll see a red question mark - just click Display to see a report of the offending transactions:

From there, you can click into the transaction and edit the tax to the correct rate. Here's a help topic that goes into a bit more detail about using the Company Data Auditor: https://www.myob.com/au/support/myob-business/accounting/financial-control/auditing-your-records/run-the-company-data-auditor 

Hope this helps! If it doesn't, or the issue is a different one, click the chat bubble on the help page and MOCA, our virtual assistant will help you out. 

Regards,

Ron