Why is Lincoln Financial saying they are due my entire SS disability payment in order to decide what to pay me?

I have been on long term disability with Lincoln Fiancial since June of 2012. I have been notified that I will get a lump sum payment from my SSI insurance dating back to June of 2012. Several time since my disability payments began the Lincoln Financial claims agent has told me that I must have an annual physicians report of my disability. I have done so. Each time they asked they said unless I did the physicians report they would offset my disability payment in the amount they estimated my SSI disability payment would be. They say now that I must send them the entire government payment and let them decide what they will pay me. I still have 17 months left on their payment schedule for my disability. I believe that I should keep the payment I have received in ESCROW until they tell me what id due back to them. I have not yet asked this but they have been insistent that we must send the entire amount back to them and let them decide what I should receive. They say that the SSI payment is theirs. What is the reason for me to send the entire payment to them I know that I should get a substantial amount of this payment but am afraid after reading the complaints that they might send me much less.