What is the ruling on organizing a competition for the Holy Qur’an, with valuable prizes designated for the top ten, without disclosing the nature of these prizes?

The competition falls under the category of Ja'alah (reward for a service/prize for a competition), and knowledge of the عوض (awad - compensation/prize) is a condition for its validity. Ignorance of the عوض invalidates the contract of Ja'alah and the competition. Therefore, if the competition does not specify the عوض, then it is not permissible.