Autofilling a cell from one dateset depending on the value of another cell.

Hi all, I'm new to the forum and a beginner to excel but I hope you can help me and i hope my thread title is correct.

I have a list of items and a list of how many are in stock. I'v managed to create a drop down list in a sales sheet so that I can choose which item I have sold, however I want the cell next to it to autofill with how many were in stock less how many I just sold.

I'm sure there must be a simple solution to this however my excel abilities stop at =sum formulas unfortunately.

I have attached a simplified version of what I am trying to achieve in a worksheet for clarity.

Thank you so much for any help you can offer me. David.

That's fantastic, thank you so much :D