public static void Exec(MercatorUi.MovableControls.MovableButton clickedButton)
{
    // enter your customized code here
    MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)clickedButton.Form;
    MercatorUi.Engine.Gescom.BillingEngine billingEngine = billingForm.BillingEngine;
    if (billingEngine.ReadOnly) // indien de gebruiker de rechten niet heeft om het document te wijzigen
         return;

    int index = Api.DataGridViewCurrentRowToDataTableRowIndex(billingForm.LinesEditor.Grid);
    if (index < 0)
        return;

    if (!string.IsNullOrWhiteSpace(billingEngine.LignesVRecords[index].ID_ARTICLE))
    {
        billingEngine.LignesVRecords[index].DataRow[billingEngine.VarQ] = (double)billingEngine.LignesVRecords[index].DataRow[billingEngine.VarQ] * -1;
        billingEngine.UpdateAmounts();
    }
    else
    {
        index = index - 1;
        if (index < 0)
            return;
        if (!string.IsNullOrWhiteSpace(billingEngine.LignesVRecords[index].ID_ARTICLE))
        {
            billingEngine.LignesVRecords[index].DataRow[billingEngine.VarQ] = (double)billingEngine.LignesVRecords[index].DataRow[billingEngine.VarQ] * -1;
            billingEngine.UpdateAmounts();
        }
    }
    MercatorUi._Divers.SuperFocus(billingForm.LinesEditor);
}