auto balance or summing

I have a table for material consumption, same product with different cost and stock as per bellow

itcode  cost    stock
100       10    100
100       11    110
100       12    500
100       13    200

If I consume a quantity of 300 i need to get the result as follows

itcode  cost    stock   auto-deduct  balance_of_stock
100     10        100    100             0
100     11        110    110             0
100     12        500    90             410     

the quantity has to deduct automatically from beginning