Isaac and the Ram

We saw last week that God would provide Someone to come and crush the head of the serpent. That Someone was to come through a man named Abraham. God promised that He would make Abraham a great nation, that all the nations of the earth would be blessed through him, and that blessing would come through a child born to Abraham and his wife, Sarah. The child arrives. It's everything they could have ever wished for. So how is that promise going to come to fruition when God commands Abraham to sacrifice his son as a burnt offering? How is the head of the serpent to be crushed if there is no line from which the seed can come?