Rashi on Exodus 3:19/רש"י על שמות ג׳:י״ט

1לא יתן אתכם מלך מצרים להלך THE KING OF EGYPT WILL NOT LET YOU GO, if I do not (ולא) show him My strong hand (יד חזקה) — it is as much as to say: so long as I do not let him feel My strong hand he will not let you go.

2לא יתן — Translate this as the Targum does: he will not give leave. Similar uses of נתן followed by ל with the infinitive in sense of permitting are: (Genesis 20:6) “therefore I did not permit thee (נתתיך) to touch her”; (Genesis 31:7) “And God did not permit him (נתנו) to do me evil”. Still all these passages really have the sense of giving”. Some explain the phrase ולא ביד חזקה to signify: “He will not permit you to go, and this will not be because his hand is strong”, for (v. 20) “As soon as I stretch forth My hand I will smite the Egyptians [… and afterwards he will let you go]”. And so the Targum renders it: and not on account that his power is mighty. This explanation was told me in the name of Rabbi Jacob the son of Menachem.