The way you’re doing it sounds like you’d still end up with the disbursement amount counting towards your “general sales” which definitely doesn’t sound right. Rather than a disbursements bank account how about creating a new nominal for disbursements in the sales range (4000-4999). When you invoice your client you’d split the invoice into two lines, with your services as “general sales” and any disbursements as the new “disbursements” code. For the balancing transaction when you pay the supplier, you’d tag the money out on your current account as “something not on the list” and post it to the same 4000-range code.
There is an open feature request to enable posting sales invoices directly to balance sheet codes, you might like to add your vote there: