IDB with an award ticket
 

I have an AA domestic award ticket on what is likely to be an oversold flight. If, hypothetically, I were to be IDBd on that flight, how does the IDB compensation work? The regs say (for 4+ hours delay), 400% of the one way fare. Does that mean they would pay my IDB compensation in miles rather than dollars, since I paid miles as my fare?

As a GENERAL proposition, AA doesn't IDB very much. They look for volunteers and, if you volunteer, you get whatever amount of flight credit they are offering.

The chance that you would get IDB is extremely remote, to the point you shouldn't consider it. If you volunteer you would be paid by a voucher that can be applied towards future travel, not miles.

I wouldn't worry too much - mis-connects, no-shows, late cancellations, etc. always seem to happen and whittle down the list to a much more manageable number.

And to add, the "voluntary" part is very much an open negotiation - ask for what you want and you just might get it.

Pre-COVID, I was on an early LAX-MIA flight that was way oversold, to the tune 20+ people. I spoke with the agent early on and mentioned I would move flights for a voucher @ max value + a confirmed seat in F on the new flight (was in MCE at present). She jumped at my offer and told me that was the easiest sale she ever made, as she hated having to IDB folks and it was never a pretty scene. Made me wonder what else I could have asked for.... but it all worked out in my favor.

And yes, the voucher would be a travel credit for $$$ toward a future flight. Had an involuntary downgrade on an award flight DFW-FAT that scored me $400 in future credits. I would imagine a IDB would follow the same case in regards to flying on an award ticket.

Recent experience here:
I was flying on a miles/award ticket this past weekend and scored a $1,000 voucher (in AA voucher dollars, not miles) due to an oversell situation out of ORD.

Voluntary or involuntary, though?

OP is specifically asking about IDB. There is no negotiation there; the DOT rules say how much you get.

Of course for voluntary, it's whatever you negotiate, and whether your ticket was cash or miles is irrelevant.

In my entire life, I only remember 2 IDB situations. One was a weight/balance situation out of one of the NYC airports. This was when they gave round-trip vouchers rather than dollar vouchers, and I think I was able to haggle with them and get one that was not capacity controlled.

Second time was last month when a plane broke in RNO and they had to fly up a replacement from PHX but it wasn't big enough and they needed 20 seats. I VDBed but there were probably a dozen or so IDB's.

I thought it could only be real IDB in the event of an oversell.

If you're IDB'd on an award ticket or other ticket that doesn't specify a cash fare (such as a vacation package or other consolidator fare), the government rules are that your compensation is based off the lowest cash, check, or credit card payment charged for a ticket in the same class of service on that flight.

To be eligible for compensation, yes, the flight must be oversold. Airlines don't have to pay compensation if they involuntarily bump you for reasons other than oversales, including equipment swaps, and in the case of small aircraft, weight and balance restrictions.

Voluntary.