increasing and decreasing a value in a cell

Well I would have Current Stock in cell C3 separate from initial stock in B3,
and following event macro in worksheet module :
Private Sub Worksheet_Change(ByVal Target As Range)
Dim OldVal As Variant, NewVal As Variant
If Target.Count > 1 Then Exit Sub
Application.EnableEvents = False
Set Target = Range("B4")
NewVal = Target.Value
Application.Undo
OldVal = Target.Value
Range("C3").Value = Range("B3").Value + Range("B5").Value + NewVal + OldVal
Target.Value = NewVal
Application.EnableEvents = True
End Sub

HTH
Carim