void billingEngine_BeginningSave(object sender, MercatorUi.Engine.Gescom.BillingEngine.BeginningSaveEventArgs e)
{
    MercatorUi.Engine.Gescom.BillingEngine billingEngine = (MercatorUi.Engine.Gescom.BillingEngine)sender;
    using (MercatorSqlCommand cmd = new MercatorSqlCommand("...", e.Connection, e.DbTransaction))
    {
        ...
    }
}