This is a sticky situation, and a big reason why the family shouldn't get too far ahead. According to the Handbook (Handbook 1:14.4.9), the amount is not to be refunded. It can be applied to another missionary, sent to the stake to share with other wards, or sent to CHQ. (See 14.4.3).
There has been suggestions that the donation be re-classified as tithing, but I'm not sure how kosher that is.
Have you searched the
Help Center? Try doing a Google search and adding "site:churchofjesuschrist.org/help" to the search criteria.
So we can better help you, please edit your
Profile to include your general location.