i need help for inventory-invoice :)

HI my name is miquel..i have problem in my worksheet. i need to link my inventory worksheet to invoice. for example: i have inventory worksheet that always update everytime i input the data. I want to input item A to invoice, then item A in warehouse automatically decreased. i need this to avoid misscalculate all the item's, so when i input item A, and the sum is larger than sum in inventory,it'll show the sum. i'll atatch my invoice sample, thx in advance