[The gabbaim of a chevra (some sort of association within a larger community – the specific nature of this chevra is not clear to me) hired a sofer to write a sefer Torah. This was decided in the presence of many members of the chevra, and they set the price and payment so that he would receive a certain amount of pay after each yeri’ah (section of parchment containing several columns). They told the sofer not to use patches (to deal with rips in the parchment). The sofer wrote twelve yeri’ot and presented them to the community. Later, new gabbaim were appointed, and he wrote another two under their tenure. The prices of sofrim went down, and the new gabbaim wanted to hire other sofrim to finish the sefer. They found that there were three yeri’ot with patches. The sofer claimed that he used the patches in a manner that he did not think people would object to, as indeed only much later were complaints registered. May the sofer be replaced?]

An agreement made in public (with three members of the community present) does not even need a kinyan to make it binding (Mordechai, Bava Metzia). The Netivot (333) asks why this concept is not found broadly and reasons that it applies only regarding matters of mitzvot. The Mordechai is discussing a Torah teacher, and the Rama (Shut 50) applies it to the appointment of a community rabbi. We can similarly apply it to one who is writing a sefer Torah on behalf of a chevra. While the Mordechai mentions the involvement of the seven leaders of the community, it appears that their involvement is not a necessity in this regard. In any case, since we are discussing an association within the community, it is likely that its gabbaim have the status of the seven leaders of the community.
Another reason not to allow the switching of sofrim is that the Shach (Yoreh Deah 280:9) rules that if there is a need to redo part of a sefer Torah, efforts should be made to have the original sofer do it because a sefer Torah with different handwritings is less respectable. How then can the gabbaim willingly initiate a change which the Shach considers to be regrettable?
The main point, though, is that in general, once a worker starts a set job, he cannot be removed without due cause. The new gabbaim are bound by the commitments of the previous ones. Additionally, the sofer continued writing during the tenure of the second set of gabbaim. One should not claim that since the unit discussed was a yeri’ah, they can discontinue his employment between yeri’ot, because the normal unit for a sefer Torah is a whole sefer (see Shach ibid.). Yeriot were mentioned in the context of providing partial compensation to the sofer without having to wait until the completion of the whole sefer.
Regarding the complaint that the sofer used patches, in violation of instructions, on one level we can say that since the sofer presented many yeri’ot to the chevra and no one had complained, they accepted the quality of his work. It is admittedly not clear if we should assume they relinquished their right to complain when they received them without complaint or if the lack of protest is significant only from the time they started using it. However, since the sofer is willing to replace the yeri’ot that have patches, this complaint is not grounds for his removal.