With unilateralcontracts where acceptance is represented by performance of the certain act theofferor can normally revoke the offer any time before completion of thisperformance (Petterson v Pattberg 1928). GBP100 was paid by Susan inreturn for the promise to secure free sofas for her, which may be a collateralcontract with enforceable promise to keep open the offer of free sofas and notto revoke it. Susan in this case would not be entitled to deduction of GBP 100from the purchase price if this amount were a consideration paid in return forpromise to keep the main offer open, increasing the amount of the main contractby GBP 100 (Heilbut, Symons & Co v Buckleton 1913).
If this GBP100 was seen as a deposit however it would be regarded as a part-satisfactionof a contract price which could be retained if Susan failed to perform her sideof the bargain (Howe v Smith 1884) (Richards, 2007, p.420). This sumcould also be treated as a prepayment for the contract to purchase three-seatersofa, in which case it could be recovered if Susan was in breach, and if thereis any doubt how such payment should be treated (deposit or prepayment) thissum should be seen as a prepayment (Dies v British & InternationalMining & Finance Corporation Ltd 1939). There is more likely to be acollateral contract here with GBP 100 non-deductible from the purchase priceunder normal circumstances or a combination of collateral contract withprepayment / deposit.