Is it permissible to rely on the predominance of an assumption that the employer agrees to my taking ice cream for my family for free, without asking him again?

The fundamental principle is the impermissibility of taking another's property except with their willing consent. The original principle is that this consent is known through certainty, not through mere conjecture. Therefore, it is not permissible to rely on predominant conjecture as long as acquiring certainty is possible, because predominant conjecture in making people's property permissible is only acted upon when certainty cannot be obtained due to the permissibility [of the action]. The Sacred Law has elevated conjecture to the status of certainty in most cases where certainty is difficult to achieve, while doubt remains abrogated by consensus.

It is not permissible for one who is able to ascertain knowledge to deviate from it to mere conjecture. Predominant conjecture is evidence in matters of Sacred Law when certainty is unattainable. It has been previously stated that predominant conjecture is valid for acting upon in matters where certainty cannot be acquired. This is further affirmed by the Prophet's (peace be upon him) saying: "Leave that which makes you doubt for that which does not make you doubt," and his saying: "The lawful is clear, and the unlawful is clear, and between them are doubtful matters which many people do not know. So whoever guards himself against doubtful matters, he has indeed cleared himself in his religion and honor. And whoever falls into doubtful matters, he falls into the unlawful."