Numbers 18:15

18:15 The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, ¹  and the firstborn males of unclean animals you must redeem.

Numbers 31:30

31:30 From the Israelites’ half-share you are to take one portion out of fifty of the people, the cattle, the donkeys, and the sheep – from every kind of animal – and you are to give them to the Levites, who are responsible for the care of the Lord’s tabernacle.”

Numbers 31:47

31:47 From the Israelites’ share Moses took one of every fifty people and animals and gave them to the Levites who were responsible for the care of the Lord’s tabernacle, just as the Lord commanded Moses.

[18:15]  ¹ tn The construction uses the infinitive absolute and the imperfect tense of the verb “to redeem” in order to stress the point – they were to be redeemed. N. H. Snaith suggests that the verb means to get by payment what was not originally yours, whereas the other root גָאַל (ga’al) means to get back what was originally yours (Leviticus and Numbers [NCB], 268).