Trying to link data from cells in two sheets

I am creating an inventory control sheet for my USN League Unit. The first sheet is a “request”. It has things like Hat/Size/quantity that needs to be entered by the user. The data goes to the second sheet and is compared to “quantity on hand”, “number of items issues” and “number of items returned”. This list is populated dependent on the first sheet input.

It goes something like this:

If sheet one “A” (cell location with a drop down menu of 6.25, 6.5, 6.75, 7.0, 7.25, 7.5, 7.75) is equal to (one of the numbers listed) then the data in aother cell from the first sheet is transported to the second sheet and displays the amount deducted from supply. i.e. Size (6.5) quantity (1) deduct (1) from inventory

I.E. =IF(sheet one first cell location=“6.5” then add (sheet one second cell location “1”) to sheet 2 cell location (creating a cell that adds or subtracts numbers based on numerical sequentials).

First Sheet
Item Size Quantity
Hat 6.5 1

Second Sheet
Item Issued Returned On Hand
Hat size 6.5 1 0 19 (20-1)
Hat size 6.25 0 0 20
Hat size 6.75 0 0 20
Hat size 7.0 0 0 20
Hat size 7.25 0 0 20
Hat size 7.5 0 0 20
Hat size 7.75 0 0 20