The Collaborative International Dictionary of English v.0.48:

Excambion \Ex*cam"bi*on\, Excambium \Ex*cam"bi*um\, n. [LL.
   excambium. See Excamb.] (Scots Law)
   Exchange; barter; -- used commonly of lands.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

EXCAMBIUM. Exchange. (q.v.)