What relationship do you have between these two tables?
Is it: Parts::Parts No = Inventory::Parts No?
You probably need to define a second summary field will all the same options as sBal EXCEPT the running balance details and then use it on your Parts layout.
You could also define a calculation field in Parts as: Sum ( Inventory::cBal ) and you'd get the same total.
In some cases especially when you get large numbers of records in Inventory, a slow network such as a WAN or are using an iPad or iPhone as the client of the hosted database, you may want to define a number field in Parts with a script trigger controlled script on the in and out fields in Inventory that updates the number field with the new balance each time the value in either the in or out field is changed. OnObjectSave is a reasonable option for this. This allows you to pop up a list of Parts records and not have to wait for balances on each part number to update for each and every record in the Parts table.
Thanks! I didn't realize the sum function can work this way.
I'll try your latter me approach.