The Collaborative International Dictionary of English v.0.48:

Overpay \O`ver*pay"\, v. t. [imp. & p. p. Overpaid; p. pr. &
   vb. n. Overpaying.]
   1. To pay too much to; to reward too highly.
      [1913 Webster]

   2. To pay too much for; as, to overpay the loan balance.
      [PJC]

WordNet (r) 3.0 (2006):

overpay
    v 1: pay too much [ant: underpay]