Or HaChaim on Deuteronomy 2:5/אור החיים על דברים ב׳:ה׳

1 לא אתן…מדרך כף רגל, "I will not give you…as much as for the sole of your foot, etc." Moses meant that G'd would not allow the Israelites to traverse the land of Edom in order to impose their will upon them. However, if the people of Edom would volunteer for the Israelites to pass through their country on their way to the West Bank, G'd would not forbid them to do so. This was the reason for sending messengers to sound out the Edomites on the subject.