Iphone 3G jailbroken and have problems trying to unjailbreak it and factory unlock it.

I bought a used iphone 3G that was already jailbroken with cydia and sam, but I don't like the affect it has on the phone. I went to wipe the phone clean and pay to have it factory unlocked and now itunes gives and error because it can't downgrade to the latest iphone 3G firmware 4.2.1. My iphone now has the screen that it has when you get it asking to sync to itunes. Itunes will not sync due to the fact that there are newer firmwares than what the iphone 3g can hold.