Subtraction in the Excel - excel

Question

I would like to create an excel template for warehouse. In this template are integrated warehouse, sellings and invoices.
I already wrote a code that transfers data to sellings every time I create an invoice and click a button. I would like to integrate in my code function that can do corrections in my warehouse i.e. when I fill an invoice I put items ID, description, amount and a price. After I click a button I want the items amount in the warehouse sheet to decreased by the same amount as in invoice.
The biggest issue for me was how to write a code that takes information from invoice sheet (ID and items amount), look for exact matches in the warehouse sheet and in the warehouse sheet column "I"(stock) decrease stock with the same amount.
sheet1 - name sheet "warehouse". ID in the column "C"; stock - column "I"; data starts from row "4"
sheet3 - name sheet "invoice". ID in the merged column "A""B"(column 1); amount that I sell in the merged column "S""T""U" (column 4); data starts from row 15 as you can see in the code below.
My code that transfers data to sellings (sheet2). I need to integrate function into this code without creating another button.
Sub Button4_Click()
Dim x As Long
Dim erow As Long
'Calculate starting rows
x = 15
With Worksheets("Sellings")
erow = .Cells(.Rows.Count, 1).End(xlUp).Offset(1, 0).Row
End With
With Worksheets("Invoice")
Do While .Cells(x, 1) <> ""
'The next line copies values to Sheet2
Worksheets("Sellings").Range("A" & erow & ":Z" & erow).Value = .Range("A" & x & ":Z" & x).Value
'increment row counters
x = x + 1
erow = erow + 1
Loop
End With
End Sub