Objection 1. It would seem that men are not bound to pay first-fruits. After giving the law of the first-born the text continues (Exodus 13:9): "It shall be as a sign in thy hand," so that, apparently, it is a ceremonial precept. But ceremonial precepts are not to be observed in the New Law. Neither therefore ought first-fruits to be paid.
Objection 2. Further, first-fruits were offered to the Lord for a special favor conferred on that people, wherefore it is written (Deuteronomy 26:2-3): "Thou shalt take the first of all thy fruits... and thou shalt go to the priest that shall be in those days, and say to him: I profess this day before the Lord thy God, that I am come into the land, for which He swore to our fathers, that He would give it us." Therefore other nations are not bound to pay first-fruits.
Objection 3. That which one is bound to do should be something definite. But neither in the New Law nor in the Old do we find mention of a definite amount of first-fruits. Therefore one is not bound of necessity to pay them.
On the contrary, It is laid down (16, qu. vii, can. Decimas): "We confirm the right of priests to tithes and first-fruits, and everybody must pay them."
I answer that, First-fruits are a kind of oblation, because they are offered to God with a certain profession (Deuteronomy 26); where the same passage continues: "The priest taking the basket containing the first-fruits from the hand of him that bringeth the first-fruits, shall set it before the altar of the Lord thy God," and further on (Deuteronomy 26:10) he is commanded to say: "Therefore now I offer the first-fruits of the land, which the Lord hath given me." Now the first-fruits were offered for a special reason, namely, in recognition of the divine favor, as though man acknowledged that he had received the fruits of the earth from God, and that he ought to offer something to God in return, according to 1 Chronicles 29:14, "We have given Thee what we received of Thy hand." And since what we offer God ought to be something special, hence it is that man was commanded to offer God his first-fruits, as being a special part of the fruits of the earth: and since a priest is "ordained for the people "in the things that appertain to God" (Hebrews 5:1), the first-fruits offered by the people were granted to the priest's use." Wherefore it is written (Numbers 18:8): "The Lord said to Aaron: Behold I have given thee the charge of My first-fruits." Now it is a point of natural law that man should make an offering in God's honor out of the things he has received from God, but that the offering should be made to any particular person, or out of his first-fruits, or in such or such a quantity, was indeed determined in the Old Law by divine command; but in the New Law it is fixed by the declaration of the Church, in virtue of which men are bound to pay first-fruits according to the custom of their country and the needs of the Church's ministers.
Reply to Objection 1. The ceremonial observances were properly speaking signs of the future, and consequently they ceased when the foreshadowed truth was actually present. But the offering of first-fruits was for a sign of a past favor, whence arises the duty of acknowledgment in accordance with the dictate of natural reason. Hence taken in a general sense this obligation remains.
Reply to Objection 2. First-fruits were offered in the Old Law, not only on account of the favor of the promised land given by God, but also on account of the favor of the fruits of the earth, which were given by God. Hence it is written (Deuteronomy 26:10): "I offer the first-fruits of the land which the Lord hath given me," which second motive is common among all people. We may also reply that just as God granted the land of promise to the Jews by a special favor, so by a general favor He bestowed the lordship of the earth on the whole of mankind, according to Psalm 113:24, "The earth He has given to the children of men."
Reply to Objection 3. As Jerome says [Comment. in Ezekiel 45:13-14; cf. Cap. Decimam, de Decim. Primit. et Oblat.]: "According to the tradition of the ancients the custom arose for those who had most to give the priests a fortieth part, and those who had least, one sixtieth, in lieu of first-fruits." Hence it would seem that first-fruits should vary between these limits according to the custom of one's country. And it was reasonable that the amount of first-fruits should not be fixed by law, since, as stated above, first-fruits are offered by way of oblation, a condition of which is that it should be voluntary.
Source: Summa Theologica (New Advent)