The Japanese central bank has over a trillion dollars of US Bonds.
They will have to sell some of those bonds to cover an interest payment due this year, which is likely to cause a run on Bond rates because this is the first time they have done so.
The only alternative is to issue more Japanese bonds to cover the interest, however this is not really practical since it would cause already high interest rates to balloon in Japan. Reducing govt spending or increased taxation would cover part of the interest but the drop in tax revenue over the medium term would worsen the debt levels.
The trade agreement that Trump announced, has not, contrary to the reporting of US media, been approved by the Japanese Diet and highly unlikely to be approved, since Trump demanded Japan actually buy even more US Bonds with the non-existent money they have.
https://www.abc.net.au/news/2025-07-11/age-verification-search-engines/105516256