Torah Temimah cites Bava Kamma.
“That which he stole” — change of form acquires title; if it is still as he stole it he returns it, and if not, money alone.
Diyuk of “asher gazal” — shinui koneh.
Aging, spoilage, or change of name shifts the obligation from the object itself to its value. The thief returns the object only when it is still “as he stole it.”