phrase requests - Is there English counterpart(s) to Japanese old saying, “Present salt to your enemy.”?

We have a popular Japanese saying, “敵に塩を送る” — literally, “present (supply) salt to one's enemy”, meaning ‘play fair and square, not taking advantage of the weak point of your rival.’ It’s different from an act which serves the interest of the enemy by the basic spirit.

This proverb came from the historic episode that Uesugi Kenshin (1530–1578), the Middle Ages warlord of Echigo Country which faces to the Japan Sea, therefore is abundant with salt resources kept supplying salt to his rival, actually enemy, Takeda Shingen (1521–1573), the warlord of Kai Country, mountainous country adjacent to Echigo, which lacks in salt essential to human existence during long lasting and consuming wars that lasted eleven years (1553–1564) between them, with 5th fatal campaigns. Takeda Shingen and Uesugi Kenshin actually crossed swords once for all on the sandbar at the confluence of Chikuma River and Sai River, which is famous for the name of the showdown on the Kawanaka-jima sandbar.

However, Uesugi never shut off the supply rout of salt to Takeda for the cause of engaging fair play. He firmly believed the war should be won in the battle field, not on the sideline such as supply rout or logistics.

With that said, we use the saying, “Present salt to one's enemy” as the metonym of fair play, gentleman-ship and sportsmanship, we call it 武士道 — Bushido — in other words, Samurai Spirit sometimes, though I don't mean every Japanese has observed or observes it.

I’m curious to know if there are counterpart English sayings, maxims, or expressions portraying the meaning to the same effect.

Answer

I would think the idiom we are looking for is closer to Provide him with a fighting chance or a level playing field

As for the Don't kick a man... I would contest that the rival in the narrative might not have been down, just lacking salt... I think the saying reflects something earlier than when the enemy is down (but could possibly also be used in that context)