Updating data

Hi,
I have a Worksheet(stock) of about 30 stock sizes with quantities which from another worksheet(quote) I can check availablity using Vlookup. If the quote then becomes a sale I would like to automatically update the stock sheet quantities against the stock size sold.
Supposing "Stock" worksheet shows cell A10 is Stock no "115270" and cell B10 shows 20 in stock when I produce my quote I have no problem looking up the stock no and showing the quantity in stock on my quote worksheet say in cell D16 but if I then sell 4 and minus them from the 20 I cant seem to find a way to then update Stock sheet B10 with the new balance. Is there an opposite of Vlookup?

No, formulaically what you describe would create a circular loop. The only way for a sheet to first see how many of an item is available (VLOOKUP), then on its own sheet SELL some of that product and have that "sale" cause the original "vlookup" result to decrement would be:

a) having sheet that has the available stock do its own "search" through saved documents and amass its own calculation. This could get weird. The cells being "searched" could not themselves be ones that search the availability sheet...more circular nonsense.

b) A macro of some sort that actually changes the number in the availability sheet when it's saved. This could be weird, too, since saving the invoice decrements, what happens when you change the sale amount and save again?

You'll need to think it though pretty carefully before approaching the VB gurus here.