The Collaborative International Dictionary of English v.0.48:

Payer \Pay"er\, n.
   One who pays; specifically, the person by whom a bill or note
   has been, or should be, paid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

payer
    n 1: a person who pays money for something [syn: payer,
         remunerator]