Struggling to work this one out

Hi There,

I have been wrapping my brain around this problems for about a week now and I just cant figure it out.

I created a stock list last week which keeps a running total of stock going in and out of our warehouse. Which worked perfectly and was super awesome. (Ok probably something really basic but it served my needs)

No what the boss wants is to know what job each product has gone to and come back from, as well as the running total.

I cant quite figure out how to minus or add the in/out columns to the running stock total on the other sheet when there are multiple instances of the same product across several jobs.

I would also like to add a drop down box to column A that lists all the product we have onsite so that there is some sort of continuity with spelling etc.

Any help would be greatly appreciated.