FYI that salt proverb: “In addition, after Shingen broke with the Hōjō, there was an incident when the
Hōjō clan blocked shipment of salt supplies to
Kai Province. When Kenshin heard of Shingen's problem, he sent salt to Shingen from his own province. Kenshin commented that the Hōjō had "performed a very mean act". Kenshin added, "I do not fight with salt, but with the sword".” (From Uesugi Kenshin’s wiki page). Kenshin and Takeda Shingen had mutual respect to each other as rival warlords, and it is known that he was greatly upset by Shingen dying ahead of him from sickness.
Remember that back then salt is a necessary commodity for seasonings and preservation of food, and Kai Province (Shingen’s territory, currently Yamamashi Prefecture) is landlocked, so they have to import salt from coastal territories such as Sagami (Hojo Ujiyasu) or Echigo (Uesugi Kenshin).