Dont think this has happened, all I've done is put £40.00 gift aid on the return and then it has calculated payment of tax £10.00 ?/ doesn't make sense to me???
With Gift Aid, the taxpayer has to have paid the amount of tax that the charity claims back from the taxman. If the self-assessment has calculated that the person is not due to pay any tax (as opposed to not owing any more tax having already paid the correct amount through PAYE) then the person has to refund HMRC the amount the charity claimed.