Halliday & Matthiessen (1999: 115):
There is thus a parallel between reported propositions/proposals and quoted ones. But with offers (proposals: giving goods–&–services) there is a difference in respect of realisation. Offers, in English, are not grammaticalised in the mood system; that is, while the other categories, statements, questions and commands, have corresponding mood categories in the grammar (declarative, interrogative and imperative) offers do not. They may be realised by any of the mood categories; for example:declarative: I can do the laundry.interrogative: Shall I do the laundry?imperative: Let me do the laundry!Significantly, the indicative clauses realising offers are modulated; they select for an imperative modality of readiness or obligation.