The powers that be might one day suddenly realise their mistake and ask for the money back. This could be tough on the couple if they don't have a lot already. I would advise them to not spend it, but again this is tough if they don't have much. I think they should write to the head of the department asking once and for all if they are entitled to this money and are they ok to use it. They should keep all copies of correspondence. If then they do have to pay it back, a nominal weekly sum "should" be accepted.