Either without a gift to me, so it is a precept; or without benefit to himself, so it is a promise. See Exodus 23:15.
Related Commentaries of Exodus 34:20
Exodus 34:20
20 But the firstling of an ass thou shalt redeem with a lamb:c and if thou redeem him not, then shalt thou break his neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty.