I'm not sure whether to believe this or not. The Police wouldn't make a decision not to charge someone for a crime where they had solid evidence on the basis of their ability to pay (pay for what?).
The police cannot sentence anyone, the CPS decide whether to prosecute in a criminal case and a court decides the guilt and sentence. The CPS also decide whether to charge someone or not.
If, as the OP states, the Police have sufficient evidence of the crime and have the suspect in custody then they are required to Refer it to the CPS unless it falls under the guidelines for giving a Police Caution.
The culprit may have been a first offender and been given a Caution which now gives him a criminal record or they did not have sufficient evidence to proceed with a conviction, either way it will not be because he couldn't afford a fine and compensation.
This.

)! The car thief is on the move.