Is it permissible for a lawyer to reach an agreement with police officers working in the court for them to direct defendants to him in exchange for a percentage of his fees, knowing that the lawyer's financial circumstances are difficult and this might protect him from falling into the forbidden?

There is no impediment to agreeing with individuals to guide (or lead to) the accused in exchange for a specified fee; this is permissible and falls under the category of brokerage (simsarah). This is indicated by the words of Allah Almighty: "And for he who brings it is the burden of a camel, and I guarantee it."