The original text here had an and after the long initial subordinate if-clause and before the main clause (“then will I spare thee”). Joseph Smith removed this Hebrew-like use of and in his editing for the 1837 edition. The critical text will restore such instances. See the discussion under 1 Nephi 17:50 as well as more generally under hebraisms in volume 3.
Summary: Restore in Alma 20:24 the Hebraistic and that originally occurred after the if-clause and before the main clause (thus “if thou wilt grant that my brethren may be cast out of prison … and then will I spare thee”).