You tell your friend you have to take your important axam that I think he can understand your situation.By the way, I think you give 500RMB as red envleop is ok. In China, this time you give red envelop to him next time he'll give it back to you! This behave makes your relationship stronger.
Your friend, if as close as you say, would know you have this exam and that it must be your current priority. It is not your problem that the 'parents' scheduled all this on this date. They did this for "their" convenience not that of others.
If you feel you need to present hong bao, then send it along with a friend with your name on it or something. But as many have mentioned, if you do this, and it is from your parents, you are truly not giving them anything. The Chinese saying: Borrowing flowers to give to Buddha.
I forgot to mention. You said your parents are not in a very good economic situation. They have sent you to school and are paying for this. Don't you think that you owe your parents to do well in school and on your exam more than attending a social function?
If I was one of your parents and I heard this discussion, I would skin you alive for wasting my money if you went to the wedding and did poorly on the exam.
Show respect where respect is due, and that is to your studies and your parents. Trust me, you won't be missed at the wedding, there will be too many people there as it is.
given the stressful condition to get there i think is enough sacrifice given that you have an important exam at hand.i think he will be more that happy to see you not the red envelope thing.he knows you are still milking your parents and won't thing anything bad.you don't need to stick in 100buk and get your transport there.that is enough for a good friend.