How to keep iPhone 3Gs at iOS 4.0 after 4.2 has been released?

[maddbeats]

Hey everyone,

I have a 32gb iphone 3Gs and am in the process of selling it on the condition that I give it to the buyer unlocked. I currently have my iPhone jailbroken and running on iOS 4.0 (8A293). I am an AT&T customer so I have not had to unlock it previously.

If I do a restore, iTunes automatically downloads the new firmware, in this case 4.2 and tries to update with that leaving the phone without the ability to have an untethered jailbreak. Is there a way to upgrade to another version of iOS through iTunes? One that would allow me to be able to jailbreak and unlock the phone for my buyer? I do not mind even keeping it running at 4.0, but need to have the ability to jailbreak and unlock it.

Thanks in advance for the help!

[iYeow]

Jailbreak your 4.0 with Jailbreakme 2.0 and unlocked with ultrasn0w
OR
restore to custom firmware of 4.1 created either by Snowbreeze 2.1 or Pwnage tools 4.1.2. If you are on a PC : use either Redsn0w 0.9.6b6 to put you into Dfu mode or use ireb 4.1 RC2 to put you into dfu mode and then shift + restore to CFW of 4.1

[maddbeats]

and if i am on a Mac?

[iYeow]

Use Pwnage tools 4.1.2 to create a custom firmware and follow the instruction to put you into dfu mode then restore to this CFW of 4.1