How to Only allow numbers and a Minus "-" in a Textbox

Just add the - to your regex character group, in a position that's not making a range of characters:

private void txtDicountSettlement_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    Regex regex = new Regex("[^0-9-]+");
    e.Handled = regex.IsMatch(e.Text);
}

I think you want something like this

^[0-9-]*$

It will match any digit any time and n no of dashes and will ignore any other character

[^-]+[^0-9]+ should prevent any input that's not an integer or a negative integer.