Shekalim, Chapter 4, Mishnah 5. This mishnah addresses the question of what was done with the incense left over at the end of the year. The underlying principle is that all communal offerings brought after Rosh Chodesh Nisan must come from the new funds that have come in, and not from the old funds.
The leftover incense:
In most years the incense was produced in a quantity greater than what was needed: they would prepare three hundred and sixty five measures, corresponding to the number of days of the year for the daily burning, plus three additional measures which the Kohen Gadol used on Yom Kippur. But an ordinary year has only three hundred and fifty four days, so there was always a surplus left over (only in a leap year are there more days). The system was designed this way from the outset: in an average year there would always be leftover incense.
The problem: how is the incense renewed:
It was not possible to take the surplus and simply apply it to the incense of the new year, since the incense of the new year must be purchased after Rosh Chodesh Nisan from the new funds. A method was therefore needed that would remove the incense from the sanctity of hekdesh, and then purchase it anew with the new funds. And they wanted to do this in an indirect way, which is what is called "tzeni'a mimitzvah," so that the purchase would be carried out discreetly and quietly, and not in a blatantly commercial manner.
The wording of the mishnah and the process of removing sanctity:
The mishnah asks: "Mosar haketores mah hayu osin bah?" - the leftover incense, how did they manage to renew it? And it answers: "Mafrishin mimenah sechar ha'umanin" - they set aside from the Terumah of the Chamber ("mimenah" refers here to the Terumah of the Chamber) a sum of money for the craftsmen who had prepared the incense. This money was set aside, and by setting it aside it was in effect given to the ones who prepared the incense. And this is the order of the steps:
Transferring the money to the craftsman: They did not hand them the money physically. Rather, they performed what is called "zikkui": they had another person pick up the money on their behalf, and legally the money became the craftsman's money. The treasurer of hekdesh who handled the process still held the money in his hands, but from now on it belonged to the craftsmen. And from that moment the money of the Terumah of the Chamber was no longer hekdesh, since it served as payment of wages to the craftsman.
Transferring the sanctity of the incense onto the craftsmen's wages: They transfer the sanctity of the leftover incense onto the craftsmen's wages. Since the craftsmen's wages had become ordinary money, the sanctity now passed over to it, and the leftover incense no longer had any sanctity.
Giving the incense to the craftsmen: The leftover incense, whose sanctity had been removed, is given to the craftsmen as their wages.
Repurchasing: They then buy back the incense, which is now ordinary and belongs to the craftsman, from the new Terumah, from the new funds that have come in.
Whether the new shekels have arrived or not:
"Koneh miTerumah chadashah" - if the new shekels arrived on time, before Rosh Chodesh Nisan, the incense is bought from the new Terumah, and this is the process described above.
"Min hayeshanah" - if the new shekels had not arrived by Rosh Chodesh Nisan, it must be bought back from the funds of the old Terumah, from the money of the previous year's Terumah of the Chamber.
All of this applies only in a case where the sanctity of the incense had already been redeemed onto the money of the craftsmen's wages. But if they had not yet done so, they would refrain from this entire process, wait until the new funds arrived, and then burn the old incense without going through the whole procedure.
In summary: In this mishnah we learned how the leftover incense was renewed. Since the communal offerings brought after Rosh Chodesh Nisan come from the new funds, and every year there is leftover incense, they instituted that wages be set aside for the craftsmen, that the money be legally transferred to them, that the sanctity of the incense be transferred onto that money, that the incense be given to them as their wages, and that it then be repurchased from the new Terumah, all in an indirect and discreet manner. And if the new shekels did not arrive in time, it is bought from the old Terumah, and if its sanctity has not yet been removed, they wait and burn it as it is.