Throughout the Makos the request was for a distance of three days. Once Makas Bechoros came about Paroah sent them out unconditionaly. He just wanted them to get out.
That is why it says Vayugad Limelech Mitzraim Ki Borchim Hem. What exactly was he told? He thought that they came back without any evidence thereof? The answer is that he never thought they should come back and it didn’t dawn on him to consider them Borchim. But, as with all negetive attitudes, when someone expresses it a certain way it catches on.
Hence, Paroah was told that, “The Jews escaped!” After that he looked at it the same way, forgetting that he actually sent them out for good.