The sales posting routine already copies Sales Line data to the Sales Invoice Line table, and it uses a function called TransferFields. This works when the source field and the target field both have the same field number and data type (and I think they have to have the same name too). What I always do is simply copy/paste the field.
So, when you add a field to the Sales Line table, and you want that to be copied to the invoice line, all you need to do is copy the field from the Sales Line table, and paste it in the Sales Invoice Line table. The existing TransferFields should take care of your field automatically.
