No option to update a Supplier Invoice right before it is set to Paid?

Hi,

I have created an extension field which should hold the date of when the invoice is set to 'Paid'. The problem is that when the invoice is in status 'Paid' it is no longer editable. Hence, I want to run my script before this happens.

Is there a good way to do this?

As of now I only tried adding an Event-Before Save to update the field on the Supplier Invoice XBO but it does not work. I also tried to trigger the update before the status is set to Paid from another BO but when i try to query the specific invoice the field is not writable.

Ideas?

Regards,

Pierre