Ok guys, I'm sure this is dead simple but it's got me flummoxed. Two sheets inside a workbook
I have an inventory list on the first sheet with 272 lines. This list has 7 columns and the first column lists part numbers (made into a table), the last column H has the stock counts. 

The second, let's call it a Log, sheet is composed of 7 columns where the first column permits a drop-down list of any item and the second column has a field entry of the quantity taken. The 4th column is the date field.

What formula can I insert to make the log sheet update the master list whenever an item is taken, so that we always have an accurate count? It would be nice if There is a formula so that when someone finishes their line entry, either the line auto-saves or auto-saves after ~2 minutes, or they can press a button (I've already made a button with Developer tab) to update the changes? Just mainly looking for a good way to build in a proper count function.

Thanks in advance.