Is it permissible not to inform the one stipulating a condition that his condition is unlawful, by analogy with the action of the Messenger of Allah (peace and blessings be upon him) with Aisha when he said: "Emancipate her and stipulate allegiance for them," then clarified that "allegiance belongs to the one who emancipates"?

It is not permissible for someone who knows a condition to be void to affirm it, because religion is advice (sincerity), and because Islamic law does not permit affirming falsehood.

Regarding the story of Barirah: 1. The first interpretation: The Prophet's (peace be upon him) statement "Stipulate their loyalty" (i.e., 'al-wala'') might mean "Stipulate loyalty from them," but this is weak. 2. The second interpretation: The meaning of "Stipulate their loyalty" is to make the ruling of 'al-wala'' manifest. 3. The third interpretation: The intent was to rebuke and admonish them, because they insisted on stipulating a void condition. It is as if the Prophet (peace be upon him) said: "Do not worry, for the condition is void and rejected." 4. The fourth (and most correct) interpretation: This condition was specific to Aisha's story, and permitting it then nullifying it was more effective in breaking their habit of such conditions and deterring them from it. A minor harm might be tolerated to achieve a great benefit.

However, if one of the stipulators was deceived by being made to believe that a void condition was valid, then if he was ignorant of the ruling, he has the option [to revoke the agreement]. But if he was aware, he has no option, because he entered into it with full knowledge. This is the preponderant opinion favored by the esteemed scholar Ibn 'Uthaymeen.