Doing a sales order invoice and adding some point of sale functionality (amount tendered,  change due,  balance due, etc.).

I have an event that computes the change,  etc. that triggers based on entering the amount tendered,  changing the freight, etc. which are all part of the header object.

However,  I would like the event to automatically trigger as well if the lines change and the invoice total changes.

It doesn't appear that there is a way to trigger based on the invoice total (at least that I have found) which leaves me with attaching the event to the fields on the detail line.   But how do you reference the parent object without opening a new instance of it?   I know there is a command to access a child object.