With of the certain act the offeror can normally

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