A good tale for you to look at in this collection of stories, some tragic and some hilarious, would be Fiammetta's Tale of Tancred and the Golden Cup. You could argue that free will is what leads to Ghismonda's triumph over her father's attempts to keep her and her lover Guiscardo apart. The way that she continues to defy her father's will for her love by insisting that they die together is shown to come true when she receives her lover's heart in Tancred's cup and then places poison in it and drinks it herself and dies. Tancred thought that fate decreed her daughter a partner who was more exalted than the hummble Guiscardo, but the tragic end of Ghismonda shows the power of free will and her ability to defy the kind of life that her station and birth would have given her.
A good thesis statement you could use for this story would therefore be something like the following:
The tragic death of Ghismonda in "The Tale of Tancred and the Golden Cup" shows the triumph of free will over fate.
This would then give you the opportunity to explore and develop your response to this question through analysing the story in more depth and detail and the role that free will plays. Hope this helps!