If a person davens mincha biychidus because he doesn’t think he will make it to a minyan-and then he makes it to a minyan should he daven again a nedavah?
Answer:
There is no obligation to daven again, even after one finds a minyan.
Sources:
See Berachos 21b, which discusses praying a nedavah, a voluntary prayer, and states that for somebody who davened privately, the fact that he now comes to a tzibbur does not mean he can daven a nedava without making a special chiddush (adding something on to his prayer).
The rishonim do not understand the Gemara as teaching that he is not obligated to pray again — this, according to most rishonim, is apparently simple, and does not need a Talmudic statement to affirm. The teaching is only that there is still a need for a chiddush.
Although some rishonim do mention that a person should daven again with a minyan, this does not seem to be an accepted opinion (see Rosh, Berachos 3:16 and Hagahos Ashri).
The reason why there is no obligation is that praying with a minyan is a “hiddur” in one’s davening: It is better to fulfill one’s obligation of prayer with a minyan, and this is possibly a full obligation (as the Iggros Moshe 3:7 understands). But once one has davened there is no longer an obligation of prayer, and the obligation of davening with a tzibbur does not apply independently.
Best wishes.
