The Collaborative International Dictionary of English v.0.48:

Assignable \As*sign"a*ble\, a.
   Capable of being assigned, allotted, specified, or
   designated; as, an assignable note or bill; an assignable
   reason; an assignable quantity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assignable
    adj 1: legally transferable to the ownership of another;
           "negotiable bonds" [syn: assignable, conveyable,
           negotiable, transferable, transferrable]