This is a very specific US-related mystery I've come across in researching some Japanese history, and I wondered if anyone on Ask Historians can shed some light on it. Please excuse the wall of text that is this question, but I think laying out all the context may help in finding an answer.

In 1863-1864, there was a military confrontation between the Western powers and the Japanese domain of Choshu, known as the Shimonoseki Campaign. Choshu domain in 1863 began firing on Western ships using the Shimonoseki strait. In 1864, a coalition of British, French, Dutch, and American ships bombarded Shimonoseki, landed marines to dismantle the Choshu batteries, and soon Choshu sued for peace. The American presence in the fleet was very small and symbolic, given that the American navy was mostly engaged in the Civil War.

As part of the peace terms, Japan was charged a huge fine to pay to the four Western powers: three million dollars. This was patently unjust, because the shogunate - who were supposed to pay it - were also at odds with Choshu. At the time of the bombardment, the shogunate and Choshu were actually at war. And the whole expedition had been launched from the port of Nagasaki, so Japan had already provided the base and supplying abilities for the expedition, something that a number of Japanese patriots held against the shogunate. But due to some legalities – I’m trying to keep this shorter – the coalition held that Japan as a country owed them the debt.

These circumstances, and how they smacked of Imperial ambition, were problematic for some American politicians from the start. The money: $785,000, was a long time in actually being paid, and then there was dissension in congress over whether it should be spent or returned to Japan. No consensus was reached for many years, until in 1883, the Indemnity was returned to Japan.

While I was researching the return of the Indemnity, I came across a passage in an 1892 guidebook to Japan that identified Charles Sumner as "foremost" in the fight to return the Indemnity to Japan. I have not been able to find any particulars about Sumner’s connection to the Indemnity issue anywhere else, but I don’t feel secure in searching in this area. The reason I’m giving one random guidebook some credence is that Sumner certainly was a friend to Japan’s first ambassador to the U.S., Mori Arinori. 1871-1873. Mori absolutely idolized Sumner, who made a huge impression on Mori’s whole worldview. Mori converted to Christianity soon after coming to the U..S. Through Sumner, Mori met Joseph Henry, the first Smithsonian secretary, and they hatched a plan for the US to return the Indemnity to fund Westernized Japanese education: Mori’s dearest cause.

So, all details point to Sumner having some involvement in this cause, but I can’t find anything precisely linking him to it. This is where I’m hoping that someone on Ask Historians might have some insight.

The tour guide’s two paragraphs singling out Sumner's contribution can be found on Page 95 of Japan As We Saw It. by Robert S. Gardiner.

The best lay-out of the timeline of the Shimonoseki Indemnity’s treatment in the House and Senate is a 1919 article from The Journal of Race Development: "The Return of the Shimonoseki Indemnity" by Payson J. Treat