Hi,i

It would be easier to explain what happened if you provided more information. All you said was I descended 500 ft... how? Did you return to cruise altitude? Did you change flight control modes? Did you change cruise altitude?

I descended by dialing an V/S of 300 ft/min.I did

You put the FMS into descent mode. Next time, just use a route offset or perform the descent manually without changing the FCP altitude.