First you need to get six pieces of scrap metal.

Now go to Protector Casdin at Fort Independence and give it all to him.

Save and try to pickpocket it back until you do.

Now give him only four pieces and get caught trying to get it back.

You will now be able to keep giving it to him, even though you don't have any.

Now go to the water processing plant in Megaton and do Walter's short side quest.

When done he will buy scrap metal off you.

Since you have the outcast glitch, in effect he will just keep giving you cash.

Note that if you pick up any more scrap metal you will have to do the glitch all over again.