Is it obligatory to pay the remaining amount ($100) to a man who completed a university admission transaction for an exorbitant price ($1000), or is this amount not rightfully his and thus waived?

There is no impediment to hiring a man to complete a transaction, as long as it does not involve nullifying a right or establishing falsehood. The agreed-upon wage must be paid. However, if the man defrauds you significantly in the wage due to your ignorance of the customary wage, you have the option of annulment due to fraud (khiyar al-ghabn), and in that case, you pay the customary wage. If the work has been completed, you pay him the customary wage. If it has not been completed, he has the choice to complete it for the customary wage or to abandon it and receive the customary wage for what he has accomplished. This is what jurists have determined regarding the establishment of the option of annulment due to fraud in leasing (ijarah), similar to selling. The man is not entitled to take more than the customary wage except with your consent.