I guess, you can't assign different journals without modifying the workflow.

If you need a separate account for that, the simpliest way is to add this discount as a separate product "Discount" with negative total in SO. Then, you may configure this product's accounts and journals.

Besides, it seems that to your purposes analytic accounting would work. Have a look at this documentation page: https://www.odoo.com/documentation/user/9.0/accounting/others/analytic/usage.html