إن اشتريت عبداً فهو حرٌّ
“If I purchase a slave, he is free.”
Take the parallel case: one half-share is purchased and sold, and then the other half-share is purchased. With purchase as the condition, the act of purchasing supplies a basis for the result concerning the later share that differs from the ownership-based case.
The key distinction is between “I own” as a state and “I buy” as an act. One must not erase the distinction by treating the expressions as interchangeable in every literal context.