What - I have customers asking for this? A Negative percentage is a "Markup". Where you make something more expensive - for whatever reason that may be. There is nothing mathematically invalid about having a negative percentage.
This is a bit like the Portal thing in #798 (closed); why introduce restrictions which are arbitrary?
@lord_alan - allowing everything will have repercussion later at places where assumption breaks. So it is good to naildown the scenarios and identify changes required in the system before cooking further.
But there is nothing wrong with a negative percentage. It is not possible to add a Percentage field that accepts negative values. This is just plain-wrong.
One of the problems we found where when the % Fileds are calculated with workflows and when the % is negative then user cann't save this records, because there is a control for negative value in % field. You have develop control, spen a develop time, for a restrict that makes limitation for customers.
Negative values are now acceptable on Percentage field.
NOTE: Inventory percentage fields continue to have restriction on the value, as it impact of change is deep and need to validate against taxes and charges. Please file a separate request with use-case for it.