There's a phrase from the Gospel that's used in the liturgy -- "sed tantum dic verbo" [et sanabitur anima mea], "but only say the word" [and my soul shall be healed]. Why verbo (dative or ablative) and not verbum (accusative)?
Seems like a perfectly legitimate instrumental use of the ablative. (Presumably one could also ask someone "dic signo", for example, so it's not a completely empty modifier of dic.) If that's the case, though, then the usual English translation is a bit off (though it's hard to see how one would do better without producing very non-idiomatic English).