Is there a simply way to lock specific fields in a record once it is created and saved?

This may be necessary for values in the main form and sub-form are not changed after the record has been process.

An example would be creating a sale for $1000 and posting it to our Accounts database, but then changing the sale amount after the invoice is posted.

Any changes would only be made by manager/administrator level.

I have made an alert on the daily report which would warn if an invoice has an error, but it is far from perfect.