Lookup after manual text input in textfield of form

Hi there,

I have got a form displaying several records ("BANF Line"). The records and the form contain two references to "Item". Our "Item" has been extended with an additional (external) product number. So the Item.No is still primary key, but the external product number is unique too.

So the form has two columns for the two product numbers. The first column ("No.") uses the automatic lookup feature, but the second doesn't of course. I managed to implement a manual lookup for the external product number. Therefore the user can click on the lookup arrow, search for the corresponding item and select it. But many users already know the product number and would like it to enter it directly into the text-field. Without additional code I'm getting the message, that the external product number isn't part of the primary key, which is correct of course. But I don't want to alter "Item" and the primary key... When I'm trying to change the OnAfterInput-Trigger and putting some manual lookup-code into it, I'm getting the message, that I cannot change the database before opening a transaction. That's because I tried to validate the Item.No when finding the corresponding Item for the external product number, and Navision doesn't allow that...

Does anybody have a hint, how I manage to update the record in my form after a direct input into the text-field?