Any agreement would appear in Series II of the Official Records, but that doesn't narrow things down much.
I would question whether there was such an agreement based on the following. On 1864-08-11, W. H. Winder proposed:
I would suggest that the Federal Government supply all the Confederate prisoners with all necessary supplies of clothing, &c., taking from each prisoner a receipt for whatever supplies may thus be furnished, on which receipt his name, regiment, and company shall be stated; that some Confederate officer shall attend to the delivery and receipting and certify to their correctness; that these accounts thus made out, whenever presented to the Confederate Government, shall forthwith be paid by it in cotton at a specified price - this would be pay equivalent to gold. The Federal Government to be allowed to send supplies to the Federal prisoners, to be distributed by a Federal officer, who can take receipts from the soldiers, to be charged against their pay.
Hitchcock wrote (
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Nothing can be done of this kind without the consent of the rebel authorities, and all experience shows that no such consent can be obtained. Those authorities have systematically refused to relieve or suffer us to relieve our soldiers held as prisoners in the South. There is but one way by which relief can reach them, and that is through the success of the Federal armies.
Later, Hitchcock would suggest that Winder's proposal be considered, even though Hitchcock remained of the opinion that the Confederates wouldn't accept it, writing in part:
I thought Mr. Winder was mistaken in supposing that the rebel authorities would consent to assume the expenses attendant upon the support of their troops in our hands when they know that their soldiers in our hands receive the treatment due to prisoners of war in civilized nations, to wit, clothing, food, and shelter, with both medicines and medical attendance whenever required, free of all charge to the rebel authorities. I further thought he was mistaken in supposing that the same authorities would consent to the relief of prisoners of war in their hands when we have so many evidences that it is a part of the policy of the South to act upon the sympathies of the North by distressing Northern men in their hands, though this is done in open violation of the laws of war and of humanity. One object of the rebel authorities is, through Popular opinion in the North, to induce the Government to send a whole army of rebels to General Lee for immediate service, when they know that within a few weeks past they were indebted to us, as shown by careful statements made by the Commissary-General of Prisoners, over 34,000 men, who ought now to be delivered to us, instead of being retained, as they are, prisoners in the South.
So Hitchcock thinks that the side holding a prisoner of war is responsible for clothing the prisoner. He also thinks that the Confederacy does not expect, and would not allow, the Federal Government to provide clothing to prisoners of war held by the Confederacy. That said, negotiations surrounding prisoners of war were very contentious, so the Confederates wouldn't necessarily share Hitchcock's view of what had been agreed to.
Bringing the