This Mishnah comes to summarize the entire topic, and it opens with the words "zeh hakelal" - this is the general rule. Until now we have distinguished between one who eats Terumah unintentionally and one who eats it deliberately, and now we are given a general formula that distinguishes between two types of payment and between their laws.
"Kol hameshalem keren vachomesh - hatashlumin terumah" - whoever pays the principal and the fifth, the payment is Terumah.
This refers to any case in which there is an obligation to restore the amount of the principal and to add a fifth to it. For example: a person who unintentionally ate four figs of Terumah restores four figs - that is the principal - and adds a fifth fig - that is the chomesh. In such a scenario the Mishnah rules that "the payment is Terumah": the tithed chullin figs that he gave, all five figs that he paid in place of the four figs he ate, become Terumah, and they must be treated as Terumah in every respect.
And what is the law if the kohen wishes to forgo the payment? On this the Mishnah says: "im ratzah kohen limchol - eino mochel" - if the kohen wishes to forgo it, he cannot. Even if the kohen whose figs were eaten is willing to waive the payment and say, "I do not want your five figs," it is not in his power to do so and he has no right to forgo it. For this is not a mere monetary debt, but a rectification whose essence is bringing new Terumah back into the world together with an added fifth.
"Vechol hameshalem es hakeren ve'eino meshalem es hachomesh - hatashlumin chullin" - and whoever pays the principal but does not pay the fifth, the payment is chullin.
In contrast, in any situation where the obligation is to restore the principal alone - such as one who ate deliberately, or one who fed his children, and the like - the payment of the principal is a monetary obligation belonging to the kohen, and it does not create new Terumah out of chullin. Therefore "the payment is chullin": whatever is used for the payment remains chullin. A person who deliberately ate four figs of a kohen's Terumah returns to him four figs only, without a fifth fig, since there is no obligation of a chomesh here, and those figs remain chullin and do not become Terumah.
From this follows the law of waiver as well: "im ratzah kohen limchol - mochel" - if the kohen wishes to forgo it, he may. If the kohen says, "I do not want your four figs; you are a thief and I want nothing to do with you," he is entitled to do so, and the one who ate is exempt from returning the four figs.
It is worth emphasizing that the exemption is from the monetary restitution alone. One who ate Terumah deliberately is liable to death at the hands of Heaven, and he is not released from that by an arrangement he reached with the kohen; his account with the Almighty stands as it is, whether the kohen accepted the four figs or not. But on the monetary level, when the obligation is for the principal alone, the kohen may refuse the payment, and that is entirely his right.
In summary: The Mishnah establishes a single rule that distinguishes between two types of payment: whoever is obligated in the principal and the fifth - the payment becomes Terumah, and the kohen cannot forgo it; and whoever is obligated in the principal alone - the payment remains chullin, and the kohen may forgo it. The difference lies in the nature of the payment: a rectification that restores Terumah to the world on the one hand, as opposed to a purely monetary obligation on the other.