The Collaborative International Dictionary of English v.0.48:

Guaranty \Guar"an*ty\, v. t. [imp. & p. p. Guarantied; p. pr.
   & vb. n. Guarantying.] [From Guaranty, n.]
   In law and common usage: To undertake or engage that another
   person shall perform (what he has stipulated); to undertake
   to be answerable for (the debt or default of another); to
   engage to answer for the performance of (some promise or duty
   by another) in case of a failure by the latter to perform; to
   undertake to secure (something) to another, as in the case of
   a contingency. See Guarantee, v. t.
   [1913 Webster]

   Note: Guaranty agrees in form with warranty. Both guaranty
         and guarantee are well authorized by legal writers in
         the United States. The prevailing spelling, at least
         for the verb, is guarantee.
         [1913 Webster]