What should a girl do who vowed to send a box of chocolates to her university professor if she succeeded, and she did succeed, but the professor did not take it?

If what was issued from you was merely an intention or a narration, it is not considered a vow and does not necessitate fulfillment. However, if you committed to giving charity to the teacher or offering a gift to him as an act of drawing closer to Allah, then it is a vow that must be fulfilled in the manner you intended. If the teacher refuses to accept it, you must give it to the poor and needy.

It must be noted that contingent vows are disliked and ought not to be made, for the Prophet, peace and blessings be upon him, said: "Indeed, it does not bring forth good, but rather it extracts from the miser."

And if the vow made to the teacher upon success was intended as a bribe, then it is not binding because it is a sin and not an act of obedience.