Hi , i just bought an iPod to a guy but he doesn't remember the code ( 4 numbers ) how can i recover it ?

If it's just the 4 digit code, see here: If you forgot the passcode on your iPod touch, or your iPod touch is disabled - https://support.apple.com/en-us/HT211080

If he doesn't remember the code to get into the iPod you should ask him for your money back.

Did you not check to see if it was working before you gave him money? I hate to sound unsympathetic, but if you didn't, why not?

You have lost your money!