A very good question,here is the answer.
First thing about the future as to Buddhism is that it is not completely fixed or completely open.
Here's an example...
If you toss a coin you know
- It is going to fall down to earth
- It could give either "Head" or "Tail"
The coin falling down is the fixed future,the coin is influenced by a natural force that we call gravity.So without any doubt and even before it happens you can say that the coin is going to fall.This is the technique used by Lord Buddha when predicting fixed future (Looking at the variables of an event an use logic and the reach of Buddha's mind to predict the conclusion).
But you can't predict the side of the coin whether it is going to be "Head" or "Tail".Now you see some room for action.This is when a Lord Buddha change the future events.When it is possible to make a positive change in a chain of events.
Let me provide some examples to the above (A) and (B) to make things more clear.
Example for (A)
If you see the story of "Devadatta thero" (the one who wanted to harm the life of Lord Buddha) you see a very good example for (A).
One day the question was asked from lord Buddha "Why Lord Buddha didn't helped Devadatta thero to become better."
Lord Buddha said "I wanted to help,i looked for any opportunity tosave him from what he would do.But it was clear that there is no helping Devadatta."
Here lord Buddha had to accept the future.
Example for (B)
When Lord Buddha saw that "Angulimaala" (The one who killed people to collect fingers) is about to kill his mother who will enter the forest to see him.Lord Buddha went there and stopped it.This event caused a nice chain reaction ("Angulimaala" became the famous "Angulimaala arahant").
This is a good example to a "Modifiable future".
So the conclusion is....
Future is not fully fixed or fully open.There is free will. But free will also make no difference (see how "Devadatta thero" time and time again to harm lord Buddha using his free will and choice,His free will did not make any difference).
Future is not predetermined.Some events are in a possibility that is so unchangeable,this is what causes the Fixed future.But some events are with certain variables that can be exploited to make a positive change in the events to occur,This makes the changeable future.
If you have any questions regarding the answer please leave a comment.
May triple gems bless you!