How To Keep DataTable's Primary Index In Sync With Database?

I'm working with a legacy database whose structure I cannot change.  I'm building a web-based editor for one of the DB's tables.  This table has a Primary Key called "Master_Idx" that is an Identity (autonumber) field.
To start, I query the DB and populate a data table which is cached.  This is used to "feed" the web form.  Any additions, changes, and deletions are reflected in this cached table.
The user then has the flexibility to press Save at any time, which ensures that the database is modified accordingly.
Here's my question:
When new records are added, I assign them a Master_Idx value, the first of which was calculated to be 1 + the Max(Master_Idx) value when the editing session started.  But when I insert these records back into the database it's possible that another user might have also been doing similar editing (of different records from the same table), so there's no guarantee that the Master_Idx values in the data table will be identical to those in the DB.  When I insert a record from the cached table back into the DB, what technique can I use to check what value was assigned to the Master_Idx?  If it's not the same as in the cached table then I need to update it locally in a few places.

I hope all of this makes sense but if it doesn't then let me know and I'll explain it in another way.

Robert W.