敵に塩を送る

The history of 敵に塩を送る。Sending salt to your enemy
敵に塩を送る [てきにしおをおくる (teki ni shio wo okuru)]

to save an enemy from trouble instead of taking advantage of their weakness, to show humanity even to one’s enemy, to help one’s enemy in difficulty

from “Imi wa“

https://en.wikipedia.org/wiki/Uesugi_Kenshin

https://en.m.wikipedia.org/wiki/Takeda_Shingen

…, after Shingen broke with Hōjō, there was an incident when the Hōjō boycotted salt supplies to Kai Province. When Kenshin heard of Shingen’s problem, he sent salt to the 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”.[12][23]

Kenshin’s respect for Shingen is evident from his reaction to Shingen’s death: he privately wept and stated, “I have lost my good rival. We won’t have a hero like that again!”[12]