I mean. They probably really did plant the money to try and paint it like it was a hired hit or something like that.
He didn’t say the gun and ID was planted, which were also found on him. I don’t see why he’d be like “yupp that murder weapon and manifesto is mine but that money sure isnt” if it wasn’t true. I mean, what does he have to lose (or gain) by lying about that?
They probably don’t trust them very well.