'gratuities are not enforceable in law. For example, this is a gratuity:
A person promises to pay you £10.
You do not offer to do anything in response.
It's a gratuitous promise. One person promises to do something but the other (you) does not promise anything. The promise to pay the £10 is not enforceable.
In legal language, "the offer to pay the £10 is not supported by consideration" or consider does not move from the promisee.
As there is no consideration provided by one party, there is no contract.'
https://hallellis.co.uk/contractual-consideration/