An eye for an eye on a figurative level -- clarity for clarity.
Bereshis Rabbah 65:5:
"When Isaac was old" (Gen. 27:1) -- R. Yitzhak began: "Who acquit the wicked for a bribe" (Isaiah 5:23), whoever accepts a bribe and justifies the wicked because of it. "The righteousness of their righteous will leave him" (ibid.), the righteousness of the righteous is Moses. Will leave him is Isaac. Because he justified the wicked one [i.e. Esau], his eyes became dim.