SURE! While, you're at it, see if you can get the neighborhood animals to urinate and defecate in it as well. Maybe rub it along the muffler of your car to get some nice pollutants in it. If need be, unzip and take a whiz in it yourself!
Anyone can tell that was sarcastic. Do not use the sand. It's likely contaminated with a wide range of things, only a few of which were listed above. Pathogens etc. If you're broke, sell your fish and buy clothes and food. You may not have a large fish budget, but that's different than being broke. 50 lbs for $3 at home depot. Are you willing to risk your fish for a few dollars value in sand?