He made a deal with the investigators, agreeing to cooperate in the investigation of Garson in exchange for a reduced charge (a plea bargain), and the promise of a positive letter from the DA to Siminovsky's sentencing judge.
In this scenario, he negotiated a deal where he would assist investigators by cooperating with their probe into Garson in exchange for a lighter sentence (reduced charge) and a positive recommendation from the DA to Siminovsky's judge.