Excel search array for matching value from data validation and then make a calculation in the adjacent cell

Sub CompareAndHighlight()
    Dim rng1 As Range, i As Long
    For i = 1 To Sheets("Sheet1").Range("D" & Rows.Count).End(xlUp).Row
        Set rng1 = Sheets("Sheet1").Range("D" & i)
        If StrComp(Trim(rng1.Text), Trim(Sheets("Sheet1").Range("H7").Text), vbTextCompare) = 0 Then
            rng1.Interior.Color = RGB(255, 255, 0)
            'Reduce quantity by quantity selected
            rng1.Offset(0, 1).Value = rng1.Offset(0, 1).Value - Sheets("Sheet1").Range("I7").Value 
        End If
        Set rng1 = Nothing
    Next i
End Sub

This version will process multiple input values in H:I, starting at row 7:

Sub UpdateInventory()
    Dim rNew As Long      'Row of new items
    Dim rTable As Long    'Row within main table
    Dim partNo As Variant 'To store part number being processed
    Dim qty As Variant    'To store new quantity

    With Worksheets("Sheet1")
        'Uncomment the following line if you want to clear out cell colouring
        'in column "D" so that it is easier to see which rows have been
        'affected by running this macro
        '.Columns("D").Interior.Color = xlNone

        For rNew = 7 To .Range("H" & .Rows.Count).End(xlUp).Row
            partNo = Trim(.Cells(rNew, "H").Text)
            qty = .Cells(rNew, "I").Value
            For rTable = 1 To .Range("D" & .Rows.Count).End(xlUp).Row
                If StrComp(Trim(.Cells(rTable, "D").Text), partNo, vbTextCompare) = 0 Then
                    'Highlight cell to show that change has occurred?
                    .Cells(rTable, "D").Interior.Color = RGB(255, 255, 0)
                    'Reduce quantity by quantity selected
                    .Cells(rTable, "E").Value = .Cells(rTable, "E").Value - qty
                    Exit For
                End If
            Next
        Next
    End With
End Sub

Note: The inner loop could be replaced with a Find. If you have a lot of data, that would be more efficient. If you don't have a lot of data (e.g. more than a couple of hundred rows) my preference would be to continue to use the loop.

To use different sheets for the drop downs and the stock list, I would use the following:

Option Explicit
Sub UpdateInventory()
    Dim wsJobCard As Worksheet
    Dim r1JobCard As Long
    Dim rJobCard As Long
    Dim colPartNoJobCard As String
    Dim colQtyJobCard As String

    Dim wsPartsList As Worksheet
    Dim r1PartsList As Long
    Dim rPartsList As Long
    Dim colPartNoPartsList As String
    Dim colQtyPartsList As String

    Dim partNo As Variant
    Dim qty As Variant

    Set wsJobCard = Worksheets("Job_Card")
    Set wsPartsList = Worksheets("Parts_List")

    'Adjust these to show which columns are being used on the two sheets
    colPartNoJobCard = "G" '????
    colQtyJobCard = "H"    '????
    colPartNoPartsList = "B"
    colQtyPartsList = "C"

    'Adjust these to show which row is the start of data on each sheet
    r1JobCard = 67
    r1PartsList = 2

    With wsPartsList
        'Uncomment the following line if you want to clear out previous
        'cell colouring so that it is easier to see which rows have been
        'affected by running this macro
        '.Columns(colPartNoPartsList).Interior.Color = xlNone

        For rJobCard = r1JobCard To wsJobCard.Range(colPartNoJobCard & wsJobCard.Rows.Count).End(xlUp).Row
            partNo = Trim(wsJobCard.Cells(rJobCard, colPartNoJobCard).Text)
            qty = wsJobCard.Cells(rJobCard, colQtyJobCard).Value
            For rPartsList = 1 To .Range(colPartNoPartsList & .Rows.Count).End(xlUp).Row
                If StrComp(Trim(.Cells(rPartsList, colPartNoPartsList).Text), partNo, vbTextCompare) = 0 Then
                    'Highlight cell to show that change has occurred?
                    .Cells(rPartsList, colPartNoPartsList).Interior.Color = RGB(255, 255, 0)
                    'Reduce quantity by quantity selected
                    .Cells(rPartsList, colQtyPartsList).Value = .Cells(rPartsList, colQtyPartsList).Value - qty
                    Exit For
                End If
            Next
        Next
    End With
End Sub