The 1874 RLDS edition added the relative pronoun that before he possessed, probably accidentally. This extra that has continued in the RLDS text up through the latest edition (1953). Spacing considerations from nearby fragments of the original manuscript support the reading without the that; there is room for the word only by supralinear insertion. Similarly, the text consistently lacks the relative pronoun in the expression “of all they possessed”:
Thus the that is not expected in Alma 13:15, and the critical text will maintain the original reading. For further discussion of the omitted relative pronoun that, see the discussion under which in volume 3.
Summary: Maintain in Alma 13:15 the original phraseology without the that in “of all he possessed”.