Numbers 3:41

3:41 And take ¹  the Levites for me – I am the Lord – instead of all the firstborn males among the Israelites, and the livestock of the Levites instead of all the firstborn of the livestock of the Israelites.”

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, ²  and the firstborn males of unclean animals you must redeem.

[3:41]  ¹ tn The verb is the perfect tense with vav (ו) consecutive; it carries forward the instructions from the preceding verse. The verb “take” now has the sense of appointing or designating the Levites.

[18:15]  ² 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).