Yes, it's possible that a campaign may end before it's been able to spend the whole budget.
The most common reason for this is that the selected audience is too small for the campaign budget and timeframe, causing the ads to stop showing.
Another reason could be that the ads stopped running due to not complying with Facebook Advertising Policies. Click here to find out more about Facebook's Advertising Policies.
As a Brand, if you're looking to re-allocate leftover advertising spend, please contact your Retailer to arrange how to manage funds.