More bug fixes

• -ve on spares point of sale,  discount to refund would not round correctly under some circumstances
• Costprice on received orders / reserved parts : if you pressed F6 to view part to be picked up, system would revert buy price to record as daily buy price, not actual cost price recorded on original receive, but only if you press F6. Now fixed so it always saves actual order receive price irrespective of what is on price/stock file.