The sale of a home in this case is reported in 2 places, form 8949 (for capital gains) and form 5405 (for the FTHB credit). Even though you qualified for the exclusion and did not have to report the capital gains, you still needed to report on form 5405.
You could file an amended return, report the sale and make the final $500 payment. However, if you paid a pro to file and they left something off, they should do the amended return for you. That would also solve the problem of you being overseas.
If you wanted to do it yourself with turbotax, you may have a problem getting a copy, since some digital downloads are blocked outside the US (I'm not sure why.). Then, you would have to first re-create your original return exactly as filed, before you can amend.
