You need a police force to enforce contracts and there is no system of exchange that can survive without contract.

No...you need a justice system to enforce contracts: contract law and dispute resolution pre-dates the idea of a police force by hundreds of years. Breaking a contract is a purely civil matter.

The police usually don't investigate tort law, only criminal law. When the police do get involved in civil law disputes it can get quite controversial.

>> contract law and dispute resolution pre-dates the idea of a police force by hundreds of years.

So does centrally controlled currency...