Legal background - IOUs/bills of exchange/promissory notes
Bills of exchange, promissory notes and IOUs are only rarely seen as debts to an estate - .although you might come across a debt of this type in connection with a loan to a family member. If the holder of a bill of exchange or of a promissory note either unconditionally renounces their rights in writing, or delivers the instrument to the person liable, this discharges the obligations of the acceptor or promissor, even though no consideration is received. So this is an exception to the rule that a unilateral discharge of a debt requires some form of consideration.
In the light of the above, an IOU for £18,000 endorsed on the back ‘£6,000 repaid by way of gift’ and signed by the deceased was agreed to be sufficient to release the £6,000 - see Edwards v Walters  2Ch pages 166 to 168.