Deuteronomy 2:7 (ESV)

7 For the LORD your God has blessed you in all the work of your hands. He knows your going through this great wilderness. These forty years the LORD your God has been with you. You have lacked nothing.”’

Israel could afford to purchase food and drink from the inhabitants of Mount Seir because the Lord had blessed his people in their work. Providing for his people for forty years in the wilderness was an act pure grace on the Lord's part, in light of their previous failures at Kadesh.