Change values while saving invoices

Hello,

This is the first time I'm on this forum, so excuse me if this topic is not at the right place ...

I search through a lot of articles in the forums, but I didn't found an answer on my question, so I ask my question here ?

I have to change the value of VBRK-ZUONR the moment an invoice is saved ... Does anyone has an idea of where I have to put this change ??

Thanks on advance,

Kurt Clement