The son of Richmond Park MP Susan Kramer broke the rules when he made a donation to the Lib Dem Party.
Jonathan Kramer made four monthly donations of £332 between December 2008 and March this year.
But the Lib Dems have had to repay the money after the Electoral Commission discovered Mr Kramer was not registered to vote in the UK.
Parties can only accept donations from those on the electoral roll.
Parties have 30 days to check whether donations are permissible or return them.
A party spokeswoman said Mr Kramer had been unaware his name had fallen off the electoral register and the money - £1,328.20 in total - had been paid back once the error was pointed out.