THIS IS FREAKING TERRIBLE TO ALL INVOLVED.  Stop the stupid work arounds and use the API to record a PROPER accounting transaction, this is an APPLICATION issue between Bill.com and QBO.  If Bill.com can't record a proper reversing entry or QBO can't enable them to do so, they shouldn't be partners that claim they work together.  QBO, fix yo stuff.  Bill.com, fix yo stuff.  Infuriating.