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You may be able to get it out with some small needlenose pliers. They make needlenose that have extremely small jaws for work in tight spaces, you may also be able to use tweezers. The needlenose would probably taper down to a smaller point, and would normally have ridged jaws for better grip. You should be able to find these at most larger parts stores, hardware or tool stores.
Another option would be to remove the buckle and turn it upside down. If it doesn't fall out on it's own then the buckle can be tapped on a table (etc) to hopefully dislodge it.
Worst case scenario would be replacing the buckle but hopefully you'll be able to get it out with one of these methods.