Moshe's terms or G-d's?

Ramban observes that when the Bnei Reuvain and Gad first consented to the terms presented by Moshe that in exchange for their taking lands in Eiver haYarden they lead the nation in the battle to conquer Eretz Yisrael, they used the expression, “avadecha ya’asu ka’asher adoni mi’tzaveh.” (32:25).  However, when Moshe then presented their agreement before Yehoshua and the leaders of the other tribes, the Bnei Reuvain and Gad responded with a slightly different expression: “asher y’daber Hashem el avadecha kein na’aseh.” (32:31).  Why the change in phraseology?

I think this answer of the Ramban is one of the biggest yesodos in the parsha:

 אמרו לו, אין אדוננו צריך לצוות עלינו בתנאי כפול, חלילה לעבדיך מעבור על מה שאדוני מצווה כי הם דברי ה' ולא נעבור על מצוותו. וזה טעם "את אשר דבר ה'", כי מתחלה אמרו (פסוק כה): כאשר אדוני מצווה:

In other words, the tribes of Reuvain and Gad first response, “ka’asher adoni mi’tzaveh,” implied that the terms to which they were agreeing were Moshe’s own.  How do you make an agreement between two parties enforceable and binding?  You go to a lawyer and draw up a contract.  Moshe proceeded to create a formal tnai kaful, putting their agreement in “legaleze” so it would carry all the force and authority of a binding contractual agreement.

Reuvain and Gad then rephrased their original consent.  “No need for all the formalities, no need for the ‘legaleze’ to get us to keep our word.”  They recognized that the terms were not Moshe’s own, but were “asher y’dabeir Hashem,” what G-d had dictated, and therefore they could not and would not back out of the deal.

The chassidishe seforim see the stylistic differences between Sefer Devarim and the other books of the Torah as a function of that sefer being a bridge between torah she’b’skav and torah sheb’al peh.  That bridge actually begins in our parsha of Matos.  Noticeably missing from the opening of our parsha is the usual declaration of, “Vayidaber Hashem el Moshe leimor...,” that we read so many times in chumash, yet, Moshe nonetheless says to the roshei hamatos, “Zeh hadavar asher tzivah Hashem…” (30:2)   This is the idea the Ramban is teaching us.  The Bnei Reuvain and Gad recognzied that the voice of Moshe, the voice of Chazal, is the ratzon Hashem.