Is it permissible to share information from a training course that the instructor stipulated, after payment of fees, should not be disseminated, even though this condition was not mentioned before registration?

We do not see any basis for this condition, as it contradicts the essence of the contract. Whoever learns something beneficial by paying a fee has the right to benefit from it. Ibn Qudamah mentioned that stipulating conditions that contradict the essence of the sale, such as not selling or gifting, are invalid conditions. These invalid conditions fall under the saying of the Prophet, peace and blessings be upon him: "Whatever condition is not in the Book of Allah is null and void, even if there were a hundred conditions." Al-Khattabi said that conditions that invalidate a sale are those that lead to ambiguity in the price, introduce uncertainty into the contract, or prevent the buyer from exercising the right of ownership. Therefore, this condition is not binding on the questioner, and there is no harm in him benefiting people with what he has learned.