Помогите мне, пожалуйста!

[eisenheim.livejournal.com]

 I was wondering if somone could help. I am having trouble making out this sentence's meaning:

"Начальник станции обязан возвратить пассажиру деньги за неиспользованные проездные документы и за несостоявшуюся перевозку его багажа в следующих случаях"

I kind of get the gist of the first part, and then it all falls apart: 
"The head of the station is obliged to retun money to passangers for unused trip documents and for _________at the next opportunity.
I get really mixed up with the end of the sentence and I keep changing in my head what it means and I get completely different meanings each time.

Please help me.