"And Abraham gave all that he had to Isaac" — this is the birthright. Another interpretation: What did he give him? His blessing. Therefore it says "and he gave." "And Abraham again [took a wife]." And why is this passage juxtaposed to the one above? To tell you that if a person has a grown son, he must marry him off first, and afterward he himself may take a wife. "And Abraham gave" etc. Because the Holy One, blessed be He, said to him, "For in Isaac shall seed be called to you" (Genesis 21:12), therefore he gave him all that he had.
Why Abraham Gave All That He Had to Isaac the Birthright
Midrash Aggadah, Genesis 25:5
ויתן אברהם את כל אשר לו ליצחק. זו הבכורה: ד"א מה נתן לו בירכו, לכך נאמר ויתן: ויוסף אברהם. ולמה נסמכה פרשה זו למעלה, לומר לך שאם יש לאדם בן גדול צריך שישאנו תחלה, ואחר כך ישא הוא אשה: ויתן אברהם וגו'. לפי שאמר לו הקב"ה כי ביצחק יקרא לך זרע (בראשית כא יב), לכך נתן לו כל אשר לו: