I'm a little new to FMPro. I hope this is the correct forum.

What's the best way to protect (make it read only) an individual sales order once it has been invoiced?

I have a field in the sales order table that is either empty (not invoiced) or contains a timestamp (date/time of invoicing) that can be used to test if the order has been invoiced.

I have a feeling that this could be done with read-only permissions on single records, but don't know how to do this.

Read up on FileMaker's access privileges. Privileges can be set on a per-record bases, and the ability to edit can be controlled.

Using permissions alone cannot prevent the addition of new detail records of a locked sales record.

Perhaps the best way is to have two layouts, an open-sales layout and a closed-sales layout (view-only).

The question now is how to coordinate these two layouts for the user to easily use.

From you original post I inferred that you wanted to know how to lock a record. It appears now that you want to know how to prevent related records from being created that link to a locked record.

My only suggestion is to script the creation of related records and put in a check to see if the parent record is locked.

The related records themselves can refer to the "lock" field of the master record and so prevent the editing of the related information.

Using permissions alone cannot prevent the addition of new detail records of a locked sales record.

No, but validation can.