Why would one say that he will pay evil?
R. Yonah: If someone damaged you, do not say, I will take vengeance so people will fear to damage me. If your colleague sinned, this does not permit you to sin. If he is obligated to pay you in Beis Din for damage, he will pay based on Mishpat.
Malbim: If someone did evil to you in business - he overcharged you or sold to you fake merchandise, do not say that you will pay to him with fake coins, like he did, in order to return your loss. Rather, hope to Hashem and He will save you and fill your loss.
Why does it say "Kaveh la'Shem v'Yosha Lach"?
R. Yonah: Do not fear that people will molest you when they see that you do not take vengeance. Since you refrain from taking vengeance due to Yir'as Hashem, hope to Him and He will save you.
Malbim: Refer to 20:22:1:2.
