Parashas Vayeishev · Passuk 37:28
And there passed by Midyanite merchants; and they drew and lifted up Yosef out of the pit, and sold Yosef to the Yishmaelim for twenty pieces of silver. And they brought Yosef into Egypt.
Torah Temimah points to the same Yerushalmi / midrashic cluster linking the twenty silver to the half-shekel (taba).
Sefaria — Yerushalmi Shekalim 2:3
English: Rabbi Levi said: How much fell to the share of each of them? A taba (half-shekel). Therefore the Torah commanded that each person give his shekel as a taba.
Derash on the verse (Yerushalmi / midrashic sugya) — not a formal named middah of the 13. The division of the sale price supplies the amount later fixed for the half-shekel.
Each brother’s share of the twenty silver was a half-shekel (taba). This becomes the basis for the mitzvah that every person give a half-shekel.