Merriam-Webster Collegiate Dictionary - voucher

 I. noun  Etymology: Anglo-French, summons to guarantee a title, from ~, verb  Date: circa 1523  1. an act of vouching  2.  a. a piece of supporting evidence ; proof  b. a documentary record of a business transaction  c. a written affidavit or authorization ; certificate  d. a form or check indicating a credit against future purchases or expenditures  3. a coupon issued by government to a parent or guardian to be used to fund a child's education in either a public or private school  II. transitive verb  Date: 1609  1. to establish the authenticity of  2. to prepare a ~ for  III. noun  Etymology: 1vouch + 2-er  Date: 1612 archaic one that guarantees ; surety