I found the answer with my final attempt, before I posted the question. The answer is the Treaty of Amity and Commerce Between His Majesty the King of Prussia, and the United States of America; September 10, 1785. In particular, it's article 24 of the treaty that states:
"And to prevent the destruction of prisoners of war, by sending them into distant & inclement countries, or by crouding them into close & noxious places, the two contracting parties solemnly pledge themselves to each other, & to the world, that they will not adopt any such practice; that neither will send the prisoners whom they may take from the other into the East-Indies, or any other parts of Asia or Africa, but that they shall be placed in some part of their dominions in Europe or America, in wholesome situations, that they shall not be confined in dungeons, prison-ships, nor prisons, nor be put into irons, nor bound, nor otherwise restrained in the use of their limbs; that the officers shall be enlarged on their paroles within convenient districts, & have comfortable quarters, & the common men be disposed in cantonments, open & extensive enough for air & exercise, and lodged in barracks as roomy & good as are provided by the party in whose power they are for their own troops; that the officers shall also be daily furnished by the party in whose power they are, with as many rations; & of the same articles & quality as are allowed by them, either in kind or by commutation, to officers of equal rank in their own army; & all others shall be daily furnished by them with such ration as they allow to a common soldier in their own service; the value whereof shall be paid by the other party on a mutual adjustment of accounts for the subsistance of prisoners at the close of the war; & the said accounts shall not be mingled with, or set off against any others, nor the balances due on them, be withheld as a satisfaction or reprisal for any other article, or for any other cause, real or pretended, whatever: that each party shall be allowed to keep a commissary of prisoners of their own appointment, with every separate cantonment of prisoners in pos- session of the other, which commissary shall see the prisoners as often as he pleases, shall be allowed to receive & distribute whatever comforts may be sent to them by their friends, & shall be free to make his reports in open letters to those who employ him; but if any officer shall break his parole, or any other prisoner shall escape from the limits of his cantonment, after they shall have been designated to him, such individual officer or other prisoner, shall forfeit so much of the benefit of this article as provides for his enlargement on parole or cantonment. & it is declared, that neither the pretence that war dissolves all treaties, nor any other whatever, shall be considered as annulling or suspending this & the next preceding article, but on the contrary, that the state of war is precisely that for which they are provided, & during which they are to be as sacredly observed as the most acknowledged articles in the law of nature or nations."
Source: Yale Law School's Avalon project