Quantity-based goals are incremental in nature hence when you try to update the goal progress it adds the value to the previous progress. Also, when you try to update the progress with a negative value, the system discards the "minus" symbol and takes it as a positive value and adds it to the previous progress.  

Goal owner/Assigner/Manager can always go and edit the goal progress from the Goal Details Page. Just open the goal and go to the Check-in History, you will find the Edit option there.