(based on Shoel U’Meishiv I:III:111)

[Reuven, Shimon, and Levi acted as partners to incrementally buy forests from the local nobleman in exchange for a certain amount of prepared wood. At some point, Shimon wanted to end his participation in the partnership. Reuven made complicated financial assurances to Levi to encourage him to jointly buy Shimon’s share. When Shimon put pressure on Levi to immediately give the money due from the buyout, Levi begged Reuven to make the payment, in exchange for which he would relinquish rights in the partnership. Reuven paid and set terms under which Levi could rejoin the partnership. It turned out that Reuven, who was the active partner, had withheld pertinent information from Levi about the partnership’s assets and profits. Levi now wants to cancel his mechilla (relinquishing of rights), whereas Reuven says that he cannot undo what was done.]

For a few reasons, Levi has not lost his part in the partnership. First, since the partnership was owned by both Reuven and Levi, mechilla is insufficient, but rather a kinyan is needed (see Choshen Mishpat 199 and S’ma 315:2). Mechilla is effective for rights to debts and other less concrete rights, not to ownership. Second, since Levi was not made aware of important information that would have discouraged him from such mechilla, the mechilla would be invalid due to its being b’taut (misinformed).
A third reason why the mechilla was not effective is that according to some poskim (see S’ma 12:21) oral mechilla without an act of kinyan does not work when the rights are recorded in a document on behalf of the one who ostensibly relinquished them. In this case, the contract with the nobleman, which Levi possesses, includes his name. Although the Shach (241:4) says that a document does not preclude oral mechilla, since there is doubt in the matter, Levi continues his rights due to the status quo.
We will discuss the latter idea a little more, based on the teaching of Rav Chayim Kohen, whose yahrtzeit is today [the day the responsum was written]. The reason that mechilla may not work when there is a document is that if the mechilla were indeed final, the mochel would not keep after that point a document that confirms his rights. If so, there are various cases where this logic and thus the halacha does not apply. In the case of a receipt which seems to include mechilla but the mochel holds the original document, the mechilla is valid because the whole point of a receipt is to deal with cases where the document cannot be returned, e.g., when the document is not found.
The gemara (Ketubot 104a) says that when a woman has not claimed her ketuba for 25 years, we assume there was mechilla, and this is even if she still has the document. The reason for this is that the passage of so much time is considered convincing evidence that there has been complete mechilla, in which case the existence of a document which just raises questions about the validity of the mechilla is ignored, and we continue to assume mechilla.
Based on all the reasons stated above, Levi can continue as a partner [if he fulfills the obligations this would entail].