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).
"dic verbo" is in contrast to the way the centurion explains his own method of getting things done, i.e., by commanding subordinates to do his bidding.
The centurion recognizes the superior authority of Jesus. Not only is the centurion unworthy to have him enter his house, but the authority of Jesus is such that he need only speak "word-wise" (i.e., not via a representative), and the thing will be done. Hence, Jesus recognizes the extraordinary faith of the centurion who knows that Jesus' power is not limited by space and time.