How to enter an advance to a vendor

We're going to pay sales commissions to partners as vendors (and not using payroll, which seems to have been my other option).  When I authorize the commission, an entry appears in accounts payable and I can "pay" the bill from there. 

My question is this: How should we handle advances we sometimes give to partners on future sales commissions.  The partners are our independent sales reps and sometimes they need some money up front.  Is there an elegant way to do this, and then run a report that will easily show where we stand with each partner?

Thanks,  Bob