In verse 104, 105, it was said: وَنَادَيْنَاهُ أَن يَا إِبْرَاهِيمُ قَدْ صَدَّقْتَ الرُّؤْيَا (and then We called out to him, "0 Ibrahim, you did make the dream come true." ), that is, in accomplishing the task entrusted with him in obedience to the command of Allah, he left nothing undone, in fact, did it perfectly to the letter (more likely so because in the dream too, it was shown that Sayyidna Ibrahim (علیہ السلام) is simply moving his knife to slaughter his son). Now that test was over, so he should let him go.