For the cost of a bag of sand i wouldnt risk it. How big is the tank?
If the sand is "live sand" it will have value to a saltwater aquarium keeper. You could probably get a saltwater guy to swap your sand for some new substrate suitable for freshwater. You both get something out of it that way.
If the sand is still live it will have all sorts of organisms in there that will simply die in freshwater and pollute the water. Not to mention it will have salt in there. But those organisms will be of benefit to a saltwater aquarium.
I would also recommend changing all the filter media for the same reason. There is nothing of benefit on the media for your FW
aquarium, but you risk polluting your system as these organisms die off. You could treat it all with a bleech solution i suppose, but just getting new is the safest way to go.