' Nur Zahlen (optional Komma und Punkt)
Public Sub Check_NumericKey(KeyAscii As Integer, _
  Optional bAllowDecimal As Boolean = True, _
  Optional bAllowNegative As Boolean = True, _
  Optional oText As TextBox = Nothing)
 
  Select Case KeyAscii
    ' Zahlen, Backspace und Return
    Case 48 To 57, 8, 13
 
    ' Minus für Negativ-Zahlen
    Case 45
      KeyAscii = 0
      If bAllowNegative Then
        If Not oText Is Nothing Then
          ' nur zulässig, wenn Cursor an 1. Position
          If (oText.SelStart = 0 Or oText.SelLength = Len(oText.Text)) Then
            KeyAscii = 45
          End If
        End If
      End If
 
    ' Aus Punkt wird autom. Komma
    Case 46, 44
      If bAllowDecimal Then
        If KeyAscii = 46 Then KeyAscii = 44
 
        ' nicht zulässig, falls bereits ein Komma
        ' enthalten
        If Not oText Is Nothing Then
          If InStr(oText.Text, ",") > 0 Then KeyAscii = 0
        End If
      Else
        KeyAscii = 0
      End If
 
    Case Else
      ' alle anderen Zeichen ignorieren
      KeyAscii = 0
  End Select
End Sub
' Betragseingabe formatieren
Public Function Format_Decimal(sValue As String, _
  Optional ByVal nDecimals As Integer = 2) As String
 
  Dim nBetrag As Double
 
  nBetrag = Int(Val(Replace(sValue, ",", ".")) * 10^nDecimals)
  Format_Decimal = Format$(nBetrag / 100, _
    "0." + String$(nDecimals, "0"))
End Function

Private Sub txtEuro_KeyPress(KeyAscii As Integer)
  ' Taste prüfen
  Check_NumericKey KeyAscii, , , txtEuro
End Sub
 
Private Sub txtEuro_Validate(Cancel As Boolean)
  ' Eingabe formatieren
 txtEuro.Text = Format_Decimal(txtEuro.Text, 2)
End Sub