The passage is not primarily referring to dedicating "oneself" (per the quote by Matthew Henry in the OPs original question), and so the musing
why not just donate the money or equivalent to begin with instead of
vowing oneself and then redeeming oneself
is not primarily relevant (but more on that in a moment). Verse 2 makes this clear:
(BHS) דַּבֵּ֞ר אֶל־בְּנֵ֤י יִשְׂרָאֵל֙ וְאָמַרְתָּ֣ אֲלֵהֶ֔ם
אִ֕ישׁ כִּ֥י יַפְלִ֖א נֶ֑דֶר בְּעֶרְכְּךָ֥ נְפָשֹׁ֖ת לַֽיהוָֽה׃
(NKJV) Speak to the children of Israel, and say to them: ‘When a
man consecrates by a vow certain persons to the LORD, according
to your valuation"
The context is a person (singular) who has authority over other persons (plural), be that children or servants. Servants were owned by Israel. Hebrew servants could be bought (Exo 21:2-6), daughters sold (Exo 21:7-11), and non-Hebrew men and women purchased (Lev 25:44-46). A Hebrew could sell themselves if very poor (Lev 25:39-40).
So the owner can choose to dedicate these others to YHWH, which is the context of Leviticus 27.
Which brings another point to clarify, the "Temple" was not in view yet at the writing of Leviticus. It was the Tabernacle, and the vow made to YHWH, which vow was handled by the priests. So nothing in this passage is dedicated to the Temple.
Additionally, the type of vow is a פלא vow (נֶ֫דֶר). The word in this context has been somewhat difficult to translate in the passage, as evidenced by the variety of choices English translations have made:
- makes a singular vow (KJV)
- consecrates by a vow (NKJV)
- makes a special vow (ESV; cf. NIV, makes a special vow to dedicate)
- makes a difficult vow (NASB)
According to HALOT, the word פלא in the hifil (as here) typically means to "do something wonderful," with the idea of "marvelous" or "amazing." BDB likewise indicates the same idea.1
It is unclear to me why then the translations do not simply state "make an extraordinary vow" or a "marvelous vow," since that is the idea of the word. The implication is a rare type of vow that occurs for dedication of something. However, the vow is distinct from the חֵ֫רֶם ("devoted") offering, noted in v.28-29, which is not redeemable, and must be killed.
So even if it is not "oneself" that is primarily in view, the OPs musing can be remapped to this: Why not just donate the money or equivalent to begin with instead of vowing children/servants and then redeeming them?
I think the answer resides in this idea of the "extraordinary" vow. Vows to God are often made under extreme stress, but even if so, vows are binding, as Num 30:2 states (NKJV):
If a man makes a vow to the LORD, or swears an oath to bind himself by
some agreement, he shall not break his word; he shall do according to
all that proceeds out of his mouth.
So in some cases, people vow "without thinking," and may vow possessions that they really would rather not part with. In some of these situations, God has allowed for a redemption—at a cost. Setting a redemption price for things vowed makes sense in this context.
Leviticus 7:16-18 offers some background to the "vow" (נֶ֫דֶר) offerings. There it states (NKJV):
16 But if the sacrifice of his offering is a vow or a voluntary
offering, it shall be eaten the same day that he offers his sacrifice;
but on the next day the remainder of it also may be eaten; 17 the
remainder of the flesh of the sacrifice on the third day must be
burned with fire. 18 And if any of the flesh of the sacrifice of his
peace offering is eaten at all on the third day, it shall not be
accepted, nor shall it be imputed to him; it shall be an abomination
to him who offers it, and the person who eats of it shall bear guilt.
Animals dedicated by vow were to be eaten in a manner similar to the peace offerings. But in Lev. 27, the topic includes what is done if one vows of things that cannot be eaten (i.e. anything other than clean animals that were not firstborns, which could not be part of a vow, since they were already YHWH's [v.26], i.e., they are not the person's property to vow upon to begin with). These types of offerings have value assigned, which clean animals do not get such an assignment for they cannot be exchanged (v.9-10), since they are to be eaten as part of the vow (per Lev. 7:16-18).
These extraordinary types of offerings that cannot be eaten may then be purchased back, but need not be so by the one giving them.
- People given as a vow could be redeemed. The exception for the poor (v.8) is likely to allow poor parents to redeem their child if they so chose. Probably in most cases this was done, with Samuel in 1 Samuel probably being a rare exception of actually being left unredeemed by his parents (cf. Jephthah's vow in Judges 11:30-31, 34-40).2
- Unclean animals could be redeemed (v.11-13), but if not, were to be sold by the priests (v.27). They could not be eaten.
- A house could be redeemed (v.14-15).
- A parcel of land one owned by right of family possession (i.e. the land of Israel's inheritance) could be redeemed (v.19), but if not, would become the possession of the priest (v.21).
- A parcel of land purchased, and thus not owned by right of family possession, then the priest who was to be gaining the land also had to pay a sum related to the time of the Jubal year as an offering (v.23), because upon the Jubal, the land would revert to the one who owned it by right of family possession (v.24). This was in place of devaluing the land when it was an inheritance, per v.18. This land was not redeemable by the one who vowed. It stayed in possession of the priest until the Jubal year and the return to the family inheritor.
- The tithes of the land (the seed/fruit of it) already belonged to YHWH (v.30), and so could not be part of a vow, but they could be redeemed if one so chose to (v.31), but tithes of the flock were non-redeemable (v.32-33).
The "vow" offerings were typically of edible, clean animals, to be partaken of in the gates of the place God has chosen (see Dt 12:5-7). In extraordinary times and circumstances, people would vow other items, and when done, rules had to be in place to handle them. That is what Leviticus 27 is providing.
Given the example of Hannah with Samuel (and Jephthah with his daughter), the redemption of a vow of a person was not mandatory.
Given that all other valuations in the chapter refer to redeeming, the context implies redemption is what the valuation is for with people as well, even though redemption is not specifically mentioned of them in v.1-8. However, this implication is confirmed by the reference to the contrasting חֵ֫רֶם ("devoted") offering of v.28, which even of people could not be "sold or redeemed." This contrast verifies that the pricing of v.1-8 for people was for the purpose of being redeemed or resold as servants, with the lesser price for the poor only allowed for the one who made the offering.
The valuation is thus an optional redemption price to get out of the extraordinary vow without loss of the item vowed (but with a monetary loss).
1 HALOT is Ludwig Koehler, , Walter Baumgartner, M. E. J. Richardson, and Johann Jakob Stamm, The Hebrew and Aramaic Lexicon of the Old Testament (Leiden: E.J. Brill, 1994–2000). BDB is Francis Brown, Samuel Rolles Driver, and Charles Augustus Briggs, Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (Oxford: Clarendon Press, 1977).
2 What makes Jephthah's vow difficult for interpreters is that he specifically mentioned offering the first thing to come out as a "burnt offering." So the question is whether he really did sacrifice her or not. I believe, since she, rather than one of his flock, first met him, that he handled her as he was supposed to for a vow offering, but left her unredeemed, and so she was a virgin dedicated to YHWH, serving in the tabernacle ministry all her life. He also likely offered a substitute burnt offering in her place.