Abraham did this so the Hittites couldn't ever come back and say "We gave you this land, now do this for us." Sometimes with gifts their a strings attached. We must be carful who we accept gifts from and their reasoning behind it. This happens alot in our world today, as many people have the mindset "You scratch my back, I'll scratch yours," or if I do you a favor, you will do one for me. We need to be aware of this deceitful nature because if we are doing things for personal gain and not for God, the devil is winning.
This is not to say if someone offers us a gift, we shouldn't take it. We should just be mindful of who and what it is, and if it looks like it's too good to be true, it probably is. We could then either pay for the gift or not accept it.
Lord, lease keep me mindful of th gifts you want me to receive come from your grace and not personal gain. Amen.