What is the ruling on using a receiver that decrypts channels illegally, and what is the expiation for that, given the inability to pay the high amount involved and not knowing the party to whom the money should be given?

It is not permissible to decrypt encrypted satellite channels because it infringes upon the rights of their owners. The value of the benefit must be paid to their owners, or given as charity on their behalf if they cannot be reached, unless they absolve the one who decrypted them. This compensation is obligatory if the broadcasted materials are permissible. As for forbidden channels, it is not permissible to watch them in the first place. If someone decrypts and watches them, there is no compensation due from them because a forbidden benefit has no value in Islamic law, and repentance suffices them.