Deuteronomy 21:14-17

14And it shall be, if thou shalt have no delight in her, then thou shalt let her go where she will; but thou shalt not sell her at all for money, thou shalt not make merchandise of her, because thou hast humbled her.

15If a man shall have two wives, one beloved, and another hated, and they have borne him children, both the beloved and the hated; and if the firstborn son be hers that was hated: 16Then it shall be, when he maketh his sons to inherit that which he hath, that he may not prefer the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: 17But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.1