[Tasos]

Καλησπέρα!

Γιώργο δοκίμασε:

Option Explicit

Private Sub TextBox1_AfterUpdate()
    Dim CellValue As String
    If Trim(Me.TextBox1.Text) = vbNullString Then
        Range("A1").ClearContents
    Else
        CellValue = GetValidNumber(Trim(Me.TextBox1.Text))
        If CellValue <> "" Then
            TextBox1.Text = CellValue
            Range("A1").Value = CDbl(CellValue)
        Else
            MsgBox "H μορφή αριθμού στο πεδίο αυτό δεν είναι έγκυρη !"
            SendKeys "+{TAB}"
        End If
    End If
End Sub

Private Function GetValidNumber(strValue As String) As String
    If Application.DecimalSeparator = "," Then
        strValue = Replace(strValue, ".", ",")
    Else
        strValue = Replace(strValue, ",", ".")
    End If
    If IsNumeric(strValue) Then GetValidNumber = strValue
End Function

Τάσος