How to block sales order when the quantity exceeds the quotation quantity

we want that the sales order item quantity should not exceed corresponding quataion item quantity.

to acheive this i have made a field on additional tab on quotation screen which is the concatenation and combination of quotation no and corresponding item no.this item no is done compersion with sales qorder quanties for which i have created the search help on va01 additional tab1 screen for user input.

Kindly give me suggestion how I can Block Sales order in case quantity exceeds quotation quantity.

Process: Make quotation with type z001. Give the item no. and the quantity. One number is created in additional tab B which is a combination of quotation number and corresponding item no.. Created sales order with reference to this quotation with order type zsor. In VA01 screen a Input field is attached containing the search help for giving user input and search help shows all the quotation number and item no combinations. In case the order qty exceeds quotation qty a message will appear.

regards,

rishi/anil