The Collaborative International Dictionary of English v.0.48:

Overplus \O"ver*plus\, n. [Over + L. plus more. See Plus, and
   cf. Surplus.]
   That which remains after a supply, or beyond a quantity
   proposed; surplus. --Shak.
   [1913 Webster] "The overplus of a great fortune." --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overplus
    n 1: extreme excess; "an embarrassment of riches" [syn:
         overplus, plethora, superfluity, embarrassment]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "overplus":
   balance, bonus, dividend, excess, extra, gratuity, lagniappe,
   leftover, margin, overabundance, overage, overflow, overkill,
   overmeasure, overmuch, overrun, overset, overstock, oversupply,
   plethora, plus, pourboire, redundancy, remainder, something extra,
   spare, superfluity, surfeit, surplus, surplusage, tip



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

OVERPLUS. What is left beyond a certain amount; the residue, the remainder
of a thing. The same as Surplus. (q.v.)
     2. The overplus may be certain or uncertain. It is certain, for
example, when an estate is worth three thousand dollars, and the owner
asserts it to be so in his will, and devises of the proceeds one thousand
dollars to A, one thousand dollars to B, and the overplus to C, and in
consequence of the deterioration of the estate, or from some other cause, it
sells for less than three thousand dollars, each of the legatees A, B and C
shall take one third: the overplus is uncertain where, for example, a
testator does not know the value of his estate, and gives various legacies
and the overplus to another legatee; the latter will be entitled only to
what may be left. 18 Ves. 466. See Residue; Surplus.