I'm kind of foggy on this issue so I hope someone more knowledge in this can chime in:

If I have a FFL dealer do a transfer for an AR-15 receiver, and then I send the receiver back to the manufacturer for a refund, do I have to tell my FFL dealer, the ATF, or the local police station anything? Or do I make a note that I returned the receiver to the manufacturer and keep a record of the shipping receipt or something?

You have two options...

First you could have the dealer send it back to where it came from but they will have to process it like a new trasnaction and enter it in their bound book again and log it out to the manufacturer when they send it. They will charge you for this I am sure.

Or...

You, as a private individual, can send firearms to a licensed person/business in another state so you can directly send it. This becomes hazy in regards to handguns. Since the state requires registration you should send handguns through a dealer so they may do the proper paperwork and get the gun unregistered from you.

Did you buy the receiver as a handgun or long gun?

No matter what you do always keep records.

It's an AR-15 non-pistol lower receiver (stripped).

I bought a mega lower once, and it was out of spec, so i sent it right back to them, and they sent me another one.  I think thats how you would do it, just without the part where they send you another one.

Quoted: It's an AR-15 non-pistol lower receiver (stripped).

How makes it?  Since you could probably sell it:)