Do you believe his claims that he was framed by the government due to his intention to inform on the secret service agents who he claimed were drinking? Or his outlandish JFK conspiracy theories about Chicago assassination teams including the arrest of two members? For which this is no supporting evidence. That is far out stuff. He fits a sympathetic political narrative. That is the sole reason he received the pardon. It has nothing to do with the merits of his case.
No outlandish JFK conspiracy.
A more likely scenario is that Jones and Spagnoli wanted to get a break on their counterfeiting charges. Bolden's duties with the Secret Service included rotations guarding the President and rotations on other Secret Service matters such as working with the anti-counterfeit section hunting down counterfeit rings. According to Wikipedia: "He [Bolden] was also reported to have "won two commendations for cracking counterfeiting rings".[4] According to Jet magazine: "In 1962 he ranked second in the nation in solving" counterfeit and check forging cases.[5]"
It is not beyond the realm of reasonable possibility that two big-time counterfeiters facing counterfeit felony charges might want to obtain some bargaining chips. They may have figured that they could allege that Bolden asked for a bribe, possibly because they did not like Bolden or possibly because they thought that Bolden was the easiest agent to pin a bribe allegation on, Bolden being black, counting on other agents not backing him up. Being a star witness against a "dirty agent" would be a pretty big bargaining chip.
Bolden may have thought that because he had voiced some criticism of other agents privately that agents had set him up for this, but it would seem to me to be more likely that Jones and Spagnoli set him up. It does seem rather far-fetched to think that a Secret Service Agent would try to sell a Secret Service file for $50,000 let alone to approach a leader of a counterfeit ring and just ask for it like that. $50k was a huge sum - about 10 years salary. There was no evidence that Bolden was in financial trouble and needed that kind of money.
But the really difficult thing to understand here is why anyone would think that a Secret Service investigation file would be worth anything to these counterfeiters. All it would do is give them some information about what was going on in the anti-counterfeit section of the Secret Service. Having the file would not guarantee that it was not copied and, in any event, that it would stop an investigation.