Our Mishnah describes the procedure for purchasing tokens and receiving the libations in the Beis HaMikdash.
The stages of the procedure:
With Yochanan: Whoever wished to bring libations would first go to Yochanan, who was appointed over the tokens, as we learned in the first Mishnah of our chapter. He hands over the money and receives a token in exchange.
With Achiyah: From there he would go to Achiyah, who was appointed over supplying the libations themselves - the meal offering, the wine and the oil. He hands him the token, and receives in exchange the libations that correspond to that token.
The reckoning in the evening:
In the evening, after the libations had been brought, and since evening is not a time fit for libations, the two appointees - Yochanan and Achiyah - would come together side by side and make an exchange: Achiyah takes out the various tokens he received from the people when he gave them the libations, and hands them over to Yochanan, receiving from him the money that corresponds to them.
A surplus and a shortfall:
"Hayah sham odef kesef" - if there was extra money, the additional benefit goes to hekdesh.
"Hayah sham chaser" - if there was a shortfall, Yochanan, who held the money, and who for some reason was left with less than what was needed to cover the quantity of libations that had been distributed, must pay the difference from his own home and his own pocket.
The reason for this: "Yado shel hekdesh al ha'elyonah" - the hand of hekdesh is always on top; hekdesh never loses out, and it is the people involved who pay the difference.
In summary: In this Mishnah we learned the order of purchasing the libations - money to Yochanan who was appointed over the tokens, and a token to Achiyah who was appointed over the libations - and the reckoning between them in the evening, in which surplus money goes up to hekdesh and a shortfall is collected from the appointee, since the hand of hekdesh is always on top.