assistance from experts to fixing theses codes

hi
when I search in the internet like my question . I 've found theses codes but there is something . it doesn't work
the idea of code shows message when fill values in textbox1 after fill combobox1,2,3 they should match with col A,B,C if the values are filled in textbox1,2 bigger than what are existed in col D then should message box the value is exceeded and show the real value for specific item . so if any body can fix it or alternative code it will be a great

Code:

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Function MatchingRowNumber(Brand As String, _
                          Typ As String, _
                          Origin As String) As Long
    ' 290


    Dim Fun As Long


    With Worksheet("List")
        ' search for the match here
        ' Fun interim result
    End With
    MatchingRowNumber = Fun
End Function


Private Function IsAvailable(ByVal Qty As Long) As Boolean
    ' 290


    Dim R          As Long
    Dim AvailQty    As Long


    R = MatchingRowNumber(ComboBox1.Value, _
                          ComboBox2.Value, _
                          ComboBox3.Value)
    AvailQty = List.Cells(R, "D").Value
    If AvailQty < Qty Then
        MsgBox "The quantity you entered exceeds availability." & vbCr & _
              "Available quantity is " & AvailQty & ".", _
              vbInformation, "Limited availability"
    Else
        IsAvailable = True
    End If
End Function


Private Sub TextBox1_Change()
    ' 290


    If IsAvailable(TextBox1.Value) Then
        ' continue with the program
    Else
        ' return to the textbox
        TextBox1.SetFocus
    End If
End Sub

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It's much easier to help if you attach a small sample workbook with some data and the existing macros

thanks for your interesting
there is another code . it copies data from userform to sheet INV , but I want before copy data . it should check the values are existed in COL D in sheet LIST if the item contain value into textbox1 or 2 bigger than what are existed in COL D in sheet LIST then should show message the number is exceeded and show me the real qty is available .

is there any help?

Why don't you show the user the availabe stock before (s)he can choose an amount ?
Why don't you restricht the user's choice to the available stock ?