Oznayim la'Torah #1: After suffering pangs of death - they could not see for six consecutive days, nor breathe or eat for three, ¹ his heart softened and he decided momentarily to send Yisrael out.

Oznayim la'Torah #2: Until now, Pharaoh kept to a plan - to see whether he could bear the plague that Hashem was about to send; and if not, to promise 'those suckers' that he would let Yisrael go, and the moment the plague ceased, he would simply retract. But now, when he saw that Hashem was sending plagues that paralyzed the limbs - until he was not even capable of sending for Moshe to make promises, and that the plague itself brought on suffering akin to death ² - and who knows what sort of plague was next on Hashem's list - whether he would be able to call Moshe even after the termination of the plague. That's why he decided to arrange a deal with Moshe before the next plague arrived.

Oznayim la'Torah #3: The fact that Yisrael did not take advantage of the plague of darkness to flee Egypt, led him to believe that Moshe had every intention of returning after sacrificing to Hashem, so he decided to let Yisrael go unconditionally - and to leave behind only the animals as a security - or, because total capitulation would be undignified. ³

Oznayim la'Torah: Because he saw how, after the first three plagues, which were performed by both Moshe and Aharon; a number of plagues were performed by Hashem, and Makas Barad by Moshe alone - and Pharaoh only called Aharon together with Moshe to remove the Arbeh in order to apologize for driving them out. Finally, when Makas Choshech was performed by Moshe alone, he came to the conclusion that everything depended upon Moshe, so he called Moshe to make his offer and not Aharon.

Ramban: Because Yisrael, who were shepherds by profession, owned vast flocks of sheep and herds of cattle. Consequently, he figured, they were unlikely to flee leaving all their property behind - and even if they did, he would inherit a vast fortune.

Oznayim la'Torah #1: Even though Pharaoh came to the conclusion that Yisrael would not flee, ¹ he was afraid that, during the celebrations Yisrael would sacrifice all their animals, thereby depriving him of a vast inheritance. ²

Oznayim la'Torah #2: Refer to 10:24:0.1:3.

Oznayim la'Torah: It was extremely difficult to capitulate to Moshe, ¹ so initially, to demonstrate his authority and to retain his self-respect, he refused to allow the animals to be taken out, and only at the end, did he add that the children may go. ²

Targum Yonasan: At the end of three days (when they could see again - Ibn Ezra, Bechor Shor).

Moshav Zekenim: It was during the first three days of darkness, ¹ for during the last three days, the Egyptians could not get up.

R. Chaim Paltiel: It was during the darkness. Even though the Egyptians could not get up, Pharaoh called out, and one who heard him called out... until Moshe heard and came. ²

Hadar Zekenim (to 12:1): It was Erev Pesach. ³

Moshav Zekenim: Because he thought that it was dark for Yisrael too - bearing in mind that he could not see them - in which case his Tefilah would not be effective. ¹

According to those who say that he called to Moshe only after the plague ended - refer to 10:24:151:1 - we can say that it was not feasible to call to Moshe during the plague. ²

Perhaps it was understood that he agrees to let Yisrael go on condition that Moshe prays and the plague ends, like the previous four times that he consented to let Yisrael go (PF).