1)    A man who was preoccupied with his affairs and thought that afterwards he would have time to pray, and in the end the time for praying passed, or one who was busy trying to avoid losing money and during that time the hour for praying had passed is not considered having deliberately missed the time (even though one should do one’s very best to avoid doing this) and he may pray a supplementary prayer during the following obligatory prayer. (Shulchan Aruch and Mishne Berurah 108)

2)    The additional service (Musaph) has no supplement. It follows that if the day had finished and the person has not prayed this additional service, he cannot replace it in the evening service after that day. (ibid)