Recent updated guidance from HMRC
HMRC will consider donations made from a waiver of a right to either a refund or loan repayment to be eligible for Gift Aid. For example, if someone had bought tickets for the congregation's fundraising event and the event was subsequently cancelled but the individual stated they did not wish a refund, this could become a donation eligible for Gift Aid.
Similarly, if someone had loaned the congregation money but then later decided to write off the remainder of the loan, then what was left of the loan could become a donation eligible for Gift Aid.
In both scenarios, the money can only be treated as a Gift Aid donation if there is a record of a formal waiver held by the congregation, the donor is a UK tax payer and all other Gift Aid rules are met. The donation will be considered to have been made at the date of the waiver and not the date of the original payment.
What HMRC considers to be a formal waiver depends on the amount being waived. For a small amount, an email, letter or recorded phone call is sufficient. However, for a larger amount, such as a cancelled loan, a legally enforceable document is required. The congregation must keep a copy of all documents for audit purposes.