Negative line s2

Within SAGE 50 we have four companies. One of them being in the UK  and one in the USA.

On the invoice for the UK we add discounts within the same invoice line, we just add in the % in the values field.

With the USA invoices they have to have the discount showing on a separate line. For this I am using S2 and tick the negative line to make this a negative. The only thing with S2 is the nominal code in posting details, defaults to the sale code field. Unfortunately SAGE does not allow this field to be changed. Therefore we have to rely on employees to amend this every time they use S2. 

Does anyone have any other suggestions on how we resolve this, get around, try something different?

Much appreciated.