Paul Davies. JC Smith's The Law of Contract (2018 2 ed). pp. 77-78.
This post's title is based on the teal sentence marked by the red triangle (▲).
How don't the two cases in the sentences underlined in red fulfill (▲)?
2.1. An "implied promise to pay a reasonable sum" "is made before the act is done". How isn't this consideration for the promise?
2.2. A "subsequent express promise to pay a particular sum, the consideration for which is the abandonment of the claim by the other party" contains "consideration" and thus is consideration, This promise happens before the sum is paid.
I'm uncertain if p. 102 from Anson's Law of Contract (2016 30 ed) helps.