collection

What is the name of your state? hawaii
i owed the US postal service $2181.00 back in 1990 due to overpayment of annual leave. i retired from federal service last february. last month the USPS informed the office of personnel management about the debt and started collection from my annuity payments. i am positive i have paid this in full. i wrote and explained the USPS but they want proof of payment which i do not have. that was 13 years ago. i wrote my bank but they keep their account infomation only for 7 years. i was working then in california but now i moved here in hawaii. is the statute of limitation applies here or in california? if it does, what is it? if it is past due would it be sufficient to tell them so? i tried to call several lawyers but seemed no one would take my case. what other options do i have?