It's called professional liability insurance
, the practice of many insurance companies for example
This is a normal requirement of the customer, who would like to obtain any guarantee that in case your actions (violated copyright, or made a critical error which involve losses, etc.) caused damage to the customer, he will receive any compensation for this damage from the insurance company.
However, apparently the customer did not fully understand how to protect themselves, because he wrote a General requirement, which can be formally done by providing the customer with insurance, and not to be in the red... not the agreed terms of insurance, specific risks, amount, etc. significant...
Any insurance is valid for a certain time (indefinite does not happen). Responsibility for violation of copyright is subject to the General Statute of limitations (under Russian law... the contract is subject to Russian law?) - 3 years from the moment of the violation of law (an important point when the violation of rights of third parties -- it may in 10 years to happen, if the code will still be someone to be used... accordingly work 10 + 3 years... and there is not fertile territory for litigation arises...). To obtain insurance for such a long period is not realistic, 5 years maybe, depends on the insurance. Cost, say the example of legal services - insurance 5 million. is about 5-20 Tr a year, I think in the development of the price range is approximately the same.
In practice the customer to take advantage of this insurance will be Oh how hard it is, you will need to go through seven circles of hell, courts of examination, to confirm the size of losses, etc. unpleasant the formalities and all of this in Russia, from which it will receive separate - an unforgettable pleasure (he didn't tell you that the insurance shall operate in the territory of his country, or his country should be a branch of an insurance...)
In our country, incidentally, the practice of more reliable tools for the protection of the financial interests - for example, different forms of security is generally provided in the form of Bank guarantees at any time, there is usually faster, and without the courts going on... so we have insurance, by and large, this "stockpile toilet paper" for a customer, when he will lay))))
In your case the ball is on your side of the field, to communicate with insurance, find out their rates, negotiate with the customer the conditions of the insurance that will suit you... pre-advance, take the customer you not much will protect from the fact that the customer did not run away...