WordNet (r) 3.0 (2006):

quid pro quo
    n 1: something for something; that which a party receives (or is
         promised) in return for something he does or gives or
         promises [syn: quid pro quo, quid]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "quid pro quo":
   agency, alternation, balance, ballast, battledore and shuttlecock,
   blow for blow, change, coaction, commutation,
   complementary distribution, concurrence, consideration,
   cooperation, counterbalance, counterchange, counterpoise,
   counterweight, cross fire, delegation, deputation, deputyship,
   displacement, engagement, equipoise, equivalent, exchange,
   give-and-take, interaction, interchange, intercommunication,
   intercourse, interlacing, intermeshing, intermutation, interplay,
   intertwining, interweaving, interworking, law of retaliation,
   lex talionis, like for like, makeweight, measure for measure, mesh,
   meshing, mutual admiration, mutual support, mutual transfer,
   mutuality, offset, permutation, power of attorney, reciprocality,
   reciprocation, reciprocity, replacement, representation,
   retaliation, seesaw, setoff, something for something,
   something in return, something of value, subrogation, substitution,
   supersedence, superseding, supersedure, supersession,
   supplantation, supplanting, supplantment, switch, talion,
   tit for tat, transposal, transposition, vicariousness



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

QUID PRO QUO. This phrase signifies verbatim, what for what. It is applied
to the consideration of a contract. See Co. Litt. 47, b; 7 Mann. & Gr. 998.