" Past consideration is no consideration" is the rule. If you put that whole expression in quotes into a search engine you'll find plenty of learned explanations
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There is no contract unless there is an agreement and consideration given by both sides to support it. Each side gives consideration in return for the other side so doing. Consideration is really no more than the "price" each party pays, though it may be in goods or services or money or even in surrendering a right in exchange for money and other ways that I haven't just listed.
It follows that someone cannot claim a contract simply because they have done something in the past. That's because the 'consideration', whatever it is they've done, has happened before any offer of or giving of consideration by the other side. If it were otherwise then people the world over would be delivering unwanted goods or doing services without being asked or as an act of charity and then later asking for payment, and be able to sue on a supposed "contract" !
Pre-existing consideration is a term which is not only not encountered in English law but is hard to find on a search engine anywhere
However, what can be found (by me anyway), suggests that it is a term for what might give the basis for a claim in equity. That is a case where there is no contract as such, because any consideration was past, but where it would be wholly unjust to deprive the claimant of their claim. An example might be the woman who lives in the house of another for many years and tends to them in their later years and who has been thoughout understood to be going to receive an interest in, or the right to remain in, the house on the death or removal of that person. In such a case a court might hold that she has a claim in equity even though there is no true contract.
Sorry I can't be more helpful .
