20,000 in a trip round the world, which he would not have undertaken without the gift, it seems to me that the donee has altered his position on the faith of the gift and that he is not unjustly enriched when the victim of the theft seeks restitution.
Furthermore, it appears that in these cases the action for money had and received is not usually founded upon any wrong by the third party, such as conversion; nor is it said to be a case of waiver of tort. It is founded simply on the fact that, as Lord Mansfield said, the third party cannot in conscience retain the money — or, as we say nowadays, for the third party to retain the money would result in his unjust enrichment at the expense of the owner of the money.
At present I do not wish to state the principle any less broadly than this: that the defence is available to a person whose position has so changed that it would be inequitable in all the circumstances to require him to make restitution, or alternatively to make restitution in full.