You definitely need to go back again to Lowes with your Saw and try again. Be calm, be polite and demand to see the manager if need be. Do not leave until you get your money back or the correct saw.
You're not the criminal, so you need to stand your ground and get what you deserve. You shouldn't be paying the price the the incompetence of Lowes return staff.
If it were me, I wouldn't have left until I got what I wanted/deserved, im not easily swayed when I know I'm right.
I once bought a new riding mower (John Deere) from Home Depot. loaded it on my trailer with the help of HD staff, secured it properly. Got home and the front deck hanger had snapped off, it was made of cheap pot metal. I didn't even take it off the trailer, I got back in the truck and took it back to HD. The manager argued with me for over 30 minutes that I must have damaged it putting it on or taking it off the trailer (nope). In the end they took it back and I got my $ back. No way was I paying for a riding mower that would break before it was even used.