Bastarache J. set out the following criteria for the application of the doctrine, at para. 59: (1) the parties’ dealings must
have been based on a shared assumption of fact or law: estoppel
requires manifest representation by statement or conduct creating a mutual assumption. Nevertheless, estoppel can arise out of
silence (impliedly); (2) a party must have conducted itself,
i.e., acted in reliance on such shared assumption, its actions
resulting in a change of its legal position; (3) it must also be
unjust or unfair to allow one of the parties to resile or depart
from the common assumption. The party seeking to establish
estoppel therefore has to prove that detriment will be suffered if
the other party is allowed to resile from the assumption since
there has been a change from the presumed position.
 The application judge referred to this test. He found that
the appellant did not meet the test because he did not act in
reliance on an assumption that Mary’s cheque would be honoured. There was nothing unfair in the outcome because Mary
could not give him what she did not have.
 The appellant argues that the parties shared a common
assumption that Mary had the necessary funds in her account
and that her cheque to Arlindo would be honoured.
 That may be true, but the assumption was made after
Mary’s cheque was delivered to Arlindo. Everyone assumed that
the cheque was good. Arlindo did not change his legal position
as a result of that assumption. He simply hoped that he would
receive Mary’s gift.
 In my view, the doctrine of estoppel by convention does
not assist the appellant.
“Equity will not strive officiously to defeat a gift”
 The appellant relies on the principle that “equity will not
strive officiously to defeat a gift”: see Pennington v. Waine,
 EWCA Civ. 227,  1 W.L.R. 2075, at paras. 54-59,
60-67, 115-117. This is in contradistinction to the somewhat
more familiar maxim, “equity will not assist a volunteer”.
 In support of this submission, the appellant cites Ziff,
at p. 163. Referring to T. Choithram International S.A. v.
Pagarani,  1 W.L.R. 1,  UKPC 46 (P.C.), the author
suggests that an imperfect gift, which is not perfected by a
transfer of possession to the donee, may nevertheless be effective
where the donor retains possession of the gift but makes a declaration evidencing an intention to hold the property in trust for
the donee. That is clearly not the case here.
 The other authorities cited by the appellant, the decision of
this court in Bank Leu AG v. Gaming Lottery Corp.,  O.J.