The Collaborative International Dictionary of English v.0.48:

Returnable \Re*turn"a*ble\, a.
   1. Capable of, or admitting of, being returned.
      [1913 Webster]

   2. (Law) Legally required to be returned, delivered, given,
      or rendered; as, a writ or precept returnable at a certain
      day; a verdict returnable to the court.
      [1913 Webster]

WordNet (r) 3.0 (2006):

returnable
    adj 1: that may be returned; "returnable bottles and cans";
           "this merchandise is returnable if you save the receipt"
           [ant: nonreturnable]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "returnable":
   atavistic, changeable, commutable, commutative, convertible, equal,
   equalizing, equivalent, even, exchanged, give-and-take,
   interchangeable, interchanged, mutual, permutable, reactionary,
   recessive, recidivist, recidivous, reciprocal, reciprocating,
   reciprocative, regressive, retaliatory, retrograde, retrogressive,
   retrorse, retroverse, reversible, reversional, reversionary,
   revertible, revulsionary, standard, swapped, switched, traded,
   transposed