Posting is a fraud control feature in MoneyWorks to prevent editing once the account has finalised. You cannot modify the transaction once posted. However, you can use the Cancel Transaction feature (from the Command menu) to correct the error.
By cancelling, it does not delete the transaction. Instead, it creates a reverse entry to correct the mistake.
For example, you entered a purchase invoice twice. You can use the cancel transaction method if the error is still within the current period. What the system does is that it creates a reverse transaction to correct the duplicate error.
However, if the mistake is fall in the past period, then you should consider entering a credit note or journal in the current period to correct an error made in the past.