I am trying to build an inventory spreadsheet where the first tab is the summary of inventory it shows everything that we have and the quantity we have left. The second tab is where the staff will come in a select what they are giving out and the quantity they are giving out. I have been trying to come up with a formula that searches the values in column b on sheet 2 and wherever the value is the same as the values in column a on sheet 1 and take the number that is in the adjacent cell (1 column over), subtracting from the beginning amount. This is what I have but it doesn't work consistently, PLEASE HELP!!

=B2-SUMIF('Inventory Distribution'!A2:C2000,LOOKUP(A2,'Inventory Distribution'!A2:A2000),'Inventory Distribution'!C2:C2000)

Example of spreadsheet:

Sheet1:

A B
1 Description Quantity Left
2 Baked Beans ‘This is the cell I need the formula
3 Peanut Butter I start with 20 Peanut butters, I want it to find all the “Peanut Butter” and look at the number beside them and subtract from the initial purchase quantity. Ex. 20-2-1=17
4 Green Beans


Sheet2:
A B C D
1 Date Distributed Item Quantity Distributed
2 7/11/10 Peanut Butter 2
3 7/13/10 Peanut Butter 1
4 7/14/10 Green Beans 4