Hello, is it possible to track with a validateLine function which user made the change?

You can get the current user with

N/runtime

but you can also check the line system notes (the far far right of the sublist), it usually tracks changes to lines for you.

The problem is our line history is not tracking when the amount is updated

Well if you can get validateLine to trigger when the amount is update d, then you can certainly get the current user with N/runtime and store it yourself

thanks. it stores the value after the invoice has been saved and the user edits. we also need it to store if the value is changed while the user initially creates the invoice