~tara and ~ba are interchangeable. But there is an important point to consider when to use ~tara or ~ba.
~tara is used when you set a condition for a particular event to happen, ie. it will happen only once.
Since his returning to his country will happen once (in this context), ~tara is used.
If we construct this sentence using ~ba (conditional form of the verb), we will get:
But this construction is incorrect.
Another example is:
The above sentence means [I] will not go to [this particular] party if it rains.
Ack! I dont know if i explained it well. Anyway, just remember, if the event is to occur once, ~tara is used.