The earliest text in Alma 9:28 treats every man as a plural: “every man shall reap a reward of their works”, which is then followed in subsequent clauses by a consistent use of the plural pronouns they and their. In his editing for the 1837 edition, Joseph Smith initially thought to replace the first their (in “a reward of their works”) with his, but then he realized that this change would require him to change the seven following instances of the plural they and their to the singular he and his (plus some associated verb forms). Consequently, he decided to edit every man to the overtly plural all men and leave all the instances of they and their. He did not, however, cross out the his he had written in 𝓟, but the 1837 edition nonetheless ended up with the intended emendation, “all men shall reap a reward of their works”.
The original text permitted every man to take plural pronouns, as in 2 Nephi 29:11: “I will judge the world / every man according to their works”; in the LDS text, this particular disagreement in number has been maintained, but it was removed from the RLDS text in the 1953 edition by replacing their with his. For further discussion and other examples of every man as a plural, see under 2 Nephi 29:11. The critical text will restore the original every man here in Alma 9:28.
Summary: Restore in Alma 9:28 the use of every man with the plural their: “every man shall reap a reward of their works”; such usage is found elsewhere in the original (and current) text.