How to update master details data.

Question

Hi

 I am developing big application containing accounting , inventory, sales purchase.

I have designed the DB for accounting, sales purchase, Receipt, Payments along with the respective forms.

I have done the data entry form for sales , purchase. The data is getting saved properly, everything is going correct way as required. 

Now the question is if i wish to alter any thing from sales or purchase or credit note invoice or debit note invoice(as these involve master and detail tables) then there will be changes in balance, stock, and posting accounts for that customer or supplier. How to manage this. For example if its sales for and i need to update quantity of 2 items and i need to delete 1 item. So please tell how to achieve. Also is it a good idea to keep the stock calculation on the fly or need to be updated in the table.

If any one can plz help me out in this matter i will be very thank full. I have done the receipt part as it require just one table.. 

Any efficient way to handle these things. I need quick ans as i need to implement it  immediately.

Thanks in advance