I beg the Reader not to overlook in those verses, that Balaam's permission to go, was on condition that the man called upon him again. Whereas it doth not appear, that he was again invited, but that he hastened in the morning to depart with them.
Related Commentaries of Numbers 22:20
Numbers 22:20-21
20 And God came unto Balaam at night, and said unto him, If the men come to call thee, rise up, and go with them; but yet the word which I shall say unto thee, that shalt thou do.
21 And Balaam rose up in the morning, and saddled his ass, and went with the princes of Moab.