WordNet (r) 3.0 (2006):

blank check
    n 1: freedom to do as you see fit; "many have doubts about
         giving him a free hand to attack" [syn: free hand, blank
         check]
    2: a check that has been signed but with the amount payable left
       blank [syn: blank check, blank cheque]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "blank check":
   CD, IOU, MO, acceptance, acceptance bill, ample scope,
   bank acceptance, bank check, bill, bill of draft, bill of exchange,
   carte blanche, certificate, certificate of deposit,
   certified check, check, checkbook, cheque, clearance,
   commercial paper, copyright, debenture, demand bill, demand draft,
   dispensation, draft, due bill, elbowroom, exchequer bill, favor,
   field, franchise, free course, free hand, free play, free scope,
   freedom, full authority, full power, full scope, full swing, grant,
   immunity, indulgence, latitude, leeway, letter of credit, liberty,
   license, long rope, maneuvering space, margin, money order,
   negotiable instrument, no holds barred, note, note of hand,
   open mandate, open space, paper, patent, play, postal order,
   privilege, promissory note, range, room, rope, scope, sea room,
   sight bill, sight draft, space, special favor, swing, time bill,
   time draft, tolerance, trade acceptance, treasury bill, voucher,
   warrant, way, wide berth