Although the word ‘may’ is generally interpreted as being discretionary (giving the party the freedom to decide), case law shows, at least in Great Britain, that the courts can, in an appropriate context, construe ‘may’ as imposing an obligation rather than a discretion.
Thus, for example, in Entwistle v. Dent (1848), a letter from a London merchant to his agent in China stated that “If tea is not obtainable at our limits you may invest one half of the whole proceeds in silk.” The court held that the context in which the words appeared in the letter inferred that the agents were directed to invest in silk and had no discretion in the matter.
When conferring a discretion, use additional words which leave no doubt that such discretion is intended, e.g.
“The Trustees may pay in their [absolute] discretion…” or
“The Trustees may pay if they think fit…”