Stories of the Japanese 01312 [Muromachi/Yoshimochi Period] Resolution of the Tsushima Issue

As mentioned above, while the relationship between Yoshimochi and Sejong was progressing well, the negotiations between the Tsushima Shugo, So Sadamori, and Taejong were in an extremely difficult situation.
On January 15, 1420 (intercalary month), Sejong sent Song Hui-gyeong (1376–1446) to Japan as an envoy to return the favor to Yoshimochi. He was heading to Japan with that precious Tripitaka. Song Hui-gyeong was supposed to be a person unrelated to the negotiations between Tsushima and Taejong, but he became involved in the issue of Tsushima becoming a vassal state in an unexpected way.
On February 28, while stopping over in Tsushima, Song Hui-gyeong was requested to have an audience by Soda Saemon Taro (?-1428), who was handling practical affairs on behalf of the still-young So Sadamori. When he responded to this, Soda said,
“It seems that talk of making Tsushima a vassal state of the Joseon Kingdom is progressing, but this was something the envoy spoke of on his own authority, and Tsushima is not aware of it.”
He claimed this, surprising Song Hui-gyeong.
According to Japanese historical materials, Song Hui-gyeong's attitude was extremely gentlemanly, and he is said to have stated,
“If that is the case, Taejong will likely not force Tsushima to become a vassal state. I will return to my home country and seek instructions from Taejong regarding that matter.”
Of course, Song Hui-gyeong was scheduled to have an audience with Yoshimochi after this, but the topic of Tsushima becoming a vassal state was barely avoided in his discussions with Yoshimochi.
Even so, it is strange that Shio Kaido (Shi-o-kai-to) would unilaterally acknowledge Tsushima becoming a vassal state. Was he a fake envoy, or did So Sadamori seriously consider entering under the Joseon Kingdom and then change his mind later? The truth remains a mystery.
On April 16, Song Hui-gyeong arrived at Hyogo Port, and after receiving various forms of hospitality, he had an audience with Yoshimochi on June 16. Yoshimochi must have been in a good mood, having been able to obtain the long-desired Tripitaka. Song Hui-gyeong set off on his return journey on July 22, and it is said that Taejong, who received the report in October, gave up on making Tsushima a vassal state.
After that, there are no records of the Joseon Kingdom seeking sovereignty over Tsushima. According to Joseon historical materials, on April 7 of the following year, 1421, So Sadamori's envoy, Qulian, came to Hanseong and said,
“What Shio Kaido stated is a delusion. An attack on Tsushima is synonymous with an attack on Japan itself. Therefore, whether or not to have intercourse with Joseon is being conducted after properly obtaining the understanding of Lord Yoshimochi.”
He stated this, strongly asserting that Tsushima was Japanese territory.
In the end, the Joseon Kingdom gave up on making Tsushima a vassal state surprisingly easily. Originally, Taejong's statement may have been a threat to the effect of “If you neglect the control of the Wokou, I will consider making you a vassal state,” and he may not have seriously considered actually possessing it.
At any rate, it can be said that the diplomacy with Joseon, which had been conducted separately by Japan and Tsushima, was finally unified here.