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It should be noted, in that God assured Avraham that the nation would leave with riches and also effectively God was directing the nation to receive their just compensation from the Egyptians (see T.B. Sanhedrin 91a), this act of asking for the valuables was obviously beneficial for the nation at the time. It was the nation's mindset at the time which could not see this. The argument would thus be that God should have, perhaps, challenged this mindset rather than accepting it through changing the language of His directive.