Vlookup with subtract

Excel Legends,

Column A,B and C contain of Order No, Part No and Qty. I need to workout the stock status of each order based on the raw data Column F and G contains of Part No and Stock.

Example;

The following part no A5697KT we have customer order 4 and we had a stock 8 nos, the part no appear the first order then formula return the stock 8, if second time return the same part with different order no then formula return the stock 7.

Any help would be much appreciated.

ORDER NO PART NO QTY STOCK PART NO STOCK
2845 A5697KT 1 8 A5697KT 8
2845 A7119KT 1 2 A7119KT 2
2845 A5696KT 1 3 A5696KT 3
2845 A7117KT 1 4 A7117KT 4
4891 A5697KT 1 7
4891 A7119KT 1 1
4891 A5696KT 1 2
4891 A7117KT 1 3
5421 A5697KT 1 6
5421 A7119KT 1 0
5421 A5696KT 1 1
5421 A7117KT 1 2
6821 A5697KT 1 5
6821 A7119KT 1 0
6821 A5696KT 1 0
6821 A7117KT 1 1