Access public variables in extension class using chain of command

When you wrap a method, you can also access public and protected methods
and variables of the base class.
CustInvoiceJour is a base class variable hence you will be able to access it. Eg.

[ExtensionOf(classStr(SalesConfirmJournalCreate))]
Final class SalesConfirmJournalCreate_Extension
{
protected void createJournalHeader()
{
next createJournalHeader();
//It's as simple as this:
custConfirmJour.SalesBalance = 0;
}
if gets an error, It's an old bug with variable names. Declare new one and use it:
protected void createJournalHeader()
{
next createJournalHeader();
CustConfirmJour custConfirmJourLocal =
custConfirmJour;
custConfirmJourLocal.MyField = 'blah-blah';
}
}