Hi Ann and Paul,
It depends whether you are implying a great deal of doubt or certainty.
In the case of the "Fête d'Anniversaire surprise", Bruno doesn't think he will be able to get there in time for the surprise element of the party. He doesn't think his boss will let him leave early. Implied is the fact that he knows his boss well and that he won't give him permission to do it.
In other words by using the indicative/future, you take away the doubt and it becomes (almost) a certainty that he won't make it in time. There is a very slight chance but unlikely.
" Il ne pense pas qu'il puisse être là à temps"
had been said, it sounds that there's a doubt but the possibility to make it is stronger...
Very subtle I know...
Hope this helps!