side deal
noun
a transaction between two people for their private benefit, which is subsidiary to a contract negotiated by them on behalf of the organizations they represent
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012