RE: Modify custom field on Posted Sales invoice?
Hi
Your request is not very clear
where was this custom field created?
If you have created the field in the Sales Header table, then all you have to do is assign the same ID to the field in 112, and the field will update automatically.
If, on the other hand, you need to update the Sales Header field once the invoice has been registered, you will need a customisation in Codeunit 80.

check my answer if it helped you, thanks
DAniele