Late January I filed my return and received a refund. Then my employer told me they "forgot" to tell me about some relocation expenses that would count as income, so I got a new W-2. I filed an amended return and paid what I now owed. But nowhere on the amended return did the original refund get mentioned (it did on my state return) How do I send the original refund back?
You do not need to send back your original refund.
If you prepared the amended return properly, your original refund was taken into consideration in calculating any additional tax due.
The "amount due" on your 1040-X shows the additional tax due on the relocation expenses added to your W-2. Essentially, this additional tax serves as a reduction in your original refund.