"Speak to the Children of Israel and have them take Me an offering; from every man whose heart impels him, you shall take My offering. This is offering you shall take from them: gold, silver and copper." (Ex. 25:2-3)

A person who strives to fulfill a mitzvah should not attempt to fulfill it for free…

A person who strives to fulfill a mitzvah should not attempt to fulfill it for free, without paying, as the verse states, ["Three times a year all your males should appear before G‑d in the place that He will choose: on the Festival of Matzot, the Festival of Shavuot, and the Festival of Sukkot. He shall not appear before G‑d empty-handed, but] everyone, according to what he can give, [according to the blessing that G‑d gives you.]" (Deut. 16:15-17)

When a person went on the obligatory pilgrimage to the Temple in Jerusalem three times a year, he was required to bring an offering with him. This shows clearly that one must pay when doing a mitzvah.

But if you contend that it is written, "Go! Buy and eat; go and buy wine and milk without money and without price" (Isaiah 55:1) - indicating that this is for free - the latter refers only to Torah study.1 Whoever wishes can acquire it without having to pay.

An atmosphere of impurity always rests upon that which is free….

However, when one strives to serve G‑d through action [i.e. mitzvot] it is forbidden to take without paying, because a person cannot draw an atmosphere of holiness upon himself without paying the full price, whatever is demanded, whether a lot or a little. This is because an atmosphere of impurity always rests upon that which is free and for nothing. But an atmosphere of holiness is not like that; it [is acquired] only when one pays the full price, with much intense effort, purification of one's body and one's home, and with the will of the heart and the soul. If only [every]one could earn [the privilege] of having the Holy Blessed One making His dwelling within him.2 In this regard it states, "[…have them take Me an offering] from every man" - from whoever is called "a man" [in Hebrew, "ish"] - i.e. one who has conquered his inclination to evil.

The Holy Blessed One desires him…when we see that this person delights in fulfilling a mitzvah….

This can be understood to say, "They should take Me as an offering," meaning that when a person does a mitzvah he is "taking" the Holy Blessed One. (Vayikra Rabba 30)

What is the meaning of "whose heart impels him"? This means that the Holy Blessed One desires him, as it is written "On Your behalf, my heart says…" (Psalms 27:8), and "Rock3 of my heart" (Psalms 73:26), referring to the Holy Blessed One.

Now how do we know that the Holy Blessed One desires him, and wishes to place His Sanctuary in his heart? When we see that this person's delights (see Torah Ohr in Leket Shmuel, fn. 5) in fulfilling a mitzvah and pursues the service of the Holy Blessed One with all his heart, then we know for sure that the Divine Presence [Shechinah] rests upon him. Then we must "acquire" such a person at the full price by being in his company and learning from him. (See Ketubot 111b)

Similarly, a righteous person must pursue a wicked person and acquire him at the full price [i.e. use maximum effort] in order to remove the poison from him and subdue his aspect of impurity and rectify his soul.

[Zohar II, 128a-b; translation and commentary by Moshe Miller]