OK, I understand. It's possible only with customisation.

E.G

file administrator/components/com_booking/controllers/reservation.php

there is row (82) looks like

$replacement = str_replace('{DEPOSIT}', BookingHelper::displayPrice($item->fullDeposit), $replacement);

place after

$replacement = str_replace('{OWING}', BookingHelper::displayPrice($item->fullPriceSupplements - $item->fullDeposit), $replacement);

into e-mail template add

Amount Owing: {OWING}