请尝试使用下一个代码。您可以从已有的按钮调用它，或者将其复制为相应的代码：

Sub updateQuantity()
  Dim rng As Range, ws2 As Worksheet, rngItem As Range
  
  Set rng = ActiveCell
  If rng.cells.count > 1 Then MsgBox "You need to select only ONE CELL...": Exit Sub
  If rng.column <> 1 Then MsgBox "The active cell must be in A:A column...": Exit Sub
  
  Set ws2 = rng.Parent.Next ' Set the second worksheet
                            ' in this code it is the next sheet after the one keeping active cell
                            
  Set rngItem = ws2.Range("A:A").Find(What:=rng.value, LookAt:=xlWhole) 'find the Item in the second sheet
  If Not rngItem Is Nothing Then 'if the item has been found:
     'add the existing value in I:I (found range row) with the one from C:C column (activecell row)
     ws2.cells(rngItem.Row, "I").value = ws2.cells(rngItem.Row, "I").value + rng.Offset(, 2).value
     rng.Offset(, 2).Resize(, 3).ClearContents 'clear content of C:E columns of the active cell row
  Else
     MsgBox "No any match for item """ & rng.value & """ has been found in """ & _
           ws2.name & """ sheet..."
  End If
End Sub

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如果你有一个ActiveX类型的按钮，在它的Click事件中写入updateQuantity。如果它是一个Form按钮，将上面的宏赋予它。
如果有什么不清楚的，不要犹豫，要求澄清。