Currently the system will not allow a vendor invoice line to have the quantity and net amount be of opposite sign; i.e. it won't allow a negative unit price. This should be configurable as a business decision and not a hard limitation in the system. Currently, the only suggested workaround from Microsoft is to split such a line into 2 lines, one with positive quantity and net amount and one with negative quantity and net amount such that the total quantity and net amount are of the desired values. Clearly, this is not a long term solution!