After the three days of darkness*, Par'oh calls for Moshe and makes another offer about exactly who/what can leave Egypt.
Sh'mot 10:24 (from Sefaria)
וַיִּקְרָ֨א פַרְעֹ֜ה אֶל־מֹשֶׁ֗ה וַיֹּ֙אמֶר֙ לְכוּ֙ עִבְד֣וּ אֶת־יְהוָ֔ה רַ֛ק צֹאנְכֶ֥ם וּבְקַרְכֶ֖ם יֻצָּ֑ג גַּֽם־טַפְּכֶ֖ם יֵלֵ֥ךְ עִמָּכֶֽם׃
Pharaoh then summoned Moses and said, “Go, worship the LORD! Only your flocks and your herds shall be left behind; even your children may go with you.”
But why, after the plague ends, would Par'oh have called for Moshe and made any concessions? While the plague might have been stifling, with those seated unable to stand etc., where was the damage (in fact, IIRC, it was a mass of Jews who died during this plague) that would have pushed Par'oh to agree to letting the Jews go?
And unlike in the case of many of the other plagues, Par'oh (according to this reading) didn't call for Moshe while it was going on in order to stop the plague; it stopped after 3 days - why would, with the plague gone, Par'oh THEN choose to allow anyone to leave?
*The opinion of commentators such as the Bechor Shor and the Ibn Ezra -- the Haamek Davar says it was during the darkness, using Jewish messengers.