What is the ruling on a vow that was contingent on the occurrence of an event, and it occurred, and then its maker fulfilled the vow?

If you intended a vow (nadhr (definition)) by saying "I will give charity," then it is binding, and it is obligatory to fulfill it upon success, and you have done so. However, if you did not intend a vow, then it is not binding, due to the absence of an explicit صيغة (formula) indicating commitment, and what you gave would be a voluntary charity. A conditional vow (nadhr mu'allaq) is disliked by most scholars.