New transactions during a Sybase "dump database" command
The dump process happens in 3 phases:


All data and log pages are dumped.
Data pages modified during phase 1 by non-logged operations are dumped.
Log pages are dumped again.


Everything happening until end of phase 2 are contained in the dump. But the changes performed during phase 3 will be missing. Of course phase 3 is pretty fast, so the chances are that you'll not be missing anything. You can check the state of your transaction log after the dump. If anthing is in there, then you know these transactions are not in the dump.

But of course transactions happening afterwards will not be in the dump either so I guess it's not much of an issue.

If your concern is that you dump multiple databases and the dumps might be out of sync, you should consider using quiesce database.