An on-line copy of Judge Ould's offer to the Feds can be found in the Official Records at the bottom of the following page.
http://www.ehistory.com/uscw/library/or/119/0871.cfm The Federal reply to Ould's offer is on the next page. No reason why Ould's offer was refused was given, but I suspect that something this important would have been discussed at cabinet level.
Ould published some of his offers concerning prisoners in the Confederate papers of the time. I've found them. Some of them were also reported in Northern papers, if I remember correctly.
Like I said there was a war going on and supplies, ships, etc, don't come easy.