A person who is hospitalised and unconscious receives an outstanding invoice (of small value, say less than £50) from a UK postal retailer.
A friend picks up the invoice, calls the retailer, explains the situation, and offers to settle the invoice on the spot and in full.
Whether or not this would be allowed would generally call for a more fact rich situation than the one presented in the original question, that would cast light upon why a retailer might be inclined to refuse to accept payment. Hypothetical legal questions that presume that people are acting irrationally for no good reason are generally ill posed and don't have meaningful answers.
Sometimes, there might be a legitimate privacy interest implicated if the invoicing party acknowledged a payment from a third party. For example, suppose that the invoice was for a paternity test and payment would confirm that the incapacitated person actually obtained a paternity test.
Sometimes, there are legal rights beyond payment that are implicated and the reasonableness of a refusal might hinge on those rights. For example, suppose that the invoice was for an option to keep using an oil well. Payment of the invoice by the deadline would keep the oil well operating and the land owner sending the invoice might prefer that it not be paid so that the oil well would be shut down. Quite a few contracts are structured in this way. Maybe the invoice was for the right to purchase a first edition of a book when it was finally released, for example, and not paying it would free up a copy for someone else at a price that had increased in the meantime.
Or, suppose that the invoice were for unpaid taxes and payment of the taxes would prevent property from being seized for sale by tax authorities, and the taxing authority would prefer that the invoice was not paid so that the valuable property could be liquidated.
But, it is hard to imagine that there would be any reason that an ordinary retailer with an ordinary bill would ever refuse payment, although I suppose that this might trigger an interest or penalty amount owed under the contract for late payment. If that were the case, the principal of mitigation of damages, which says that a party to a contract must take all reasonable steps to mitigate their damages, might obligate the retailers to accept the payment or forfeit the penalty amounts that the retailer could have avoided as damages by accepting payment.
I doubt that an undelivered tender of payment from a third party would eliminate the obligation, but, it might limit the damages that could be claimed as in the scenario above.
On what grounds, if any, can the retailer refuse the payment?
The third party is not privy to the contract between the retailer and consumer. This means that the contract does not legally bind the third party, and therefore any performance by the third party does not constitute performance of the contract by the consumer (in this case, the obligation to pay).
Therefore, the retailer does not have to accept payment.
There are two ways I can think of that such a third party may pay the debts of his friend:
First, he may ask the consumer for authority to pay on his behalf. If he obtains this authority, he will be acting as an agent, and therefore the third party may then pay the debt on the consumer's behalf. The retailer must accept this payment as settlement of the debt. Technically speaking, it is still the consumer who has settled the debt.
Second, the third party may make an offer to the retailer to settle the debts of the consumer. If the retailer accepts this offer (and thus accepts the payment) he will be agreeing to discharging the consumer's obligation to pay him. Rather than a variation of contract, this would probably be construed as an agreement not to enforce a contractual obligation (the obligation for the consumer to pay).
Are there any circumstances in which refusal of an offer of full payment by a third party constitutes a waiver of all or part of the debt?
I would say generally no. Perhaps exceptions exist in equity, such as if, the retailer, at some point, promised the consumer that he would accept payment by third party, and the consumer acts on this promise to his detriment (such as by taking the money he saved for payment and passing it on as a gift to the third party). The retailer might then be estopped from enforcing his claim for payment from the consumer. Though this is somewhat convoluted, it is theoretically possible.
Tendering performance is a defence to a breach of contract (ie in this case, non-payment of a debt).
If the third party was the agent of the friend (and the friend being the principal), then the principal is said to have tendered performance of the contract (ie paying the money). It's an application of the law of agency.
