Who is responsible for giving me an accounting of the overpayment owned? Can I negotiate the amount of repayment?

I received overpayment from SS as a result of SSI award. Prudential and Allsup have provided me with four different accountings of how much of that payment is to be awarded to Prudential as backpayment for LTD disability benefits. I have requested itemized account (in writing) from both of them to no avail. However, instead I receive calls every day from Allsup. If I do finally receive itemized from Prudential, can I bargain/settle with them on the award amount I believe is accurate? Is this negotiable? What assurances do I have from Prudential that the will not hold this as prejudice going forward (as they are providing me with reduced benefits that are to be reviewed early 2015 for continuation)?