@hyoksoo , assuming that you have been able to exclude all your active income ( using Foreign Earned Income exclusion ) , the only remaining issue would be FICA/SECA ( at 15.3% for Social Security & Medicare ). This can also be reduced or eliminated if you are participating in South Korea's equivalent to Social Security / Medicare. US & South Korea indeed have a totalization agreement -- thus you pay ONLY to one country. Depending on your long term plans on where you are going to live and receive Social Security benefits. You will need a certificate of p[articipation from South Korea to include on your Schedule-SE to not have to pay SECA taxes or vice versa for equivalent action in SK.
I have read through the responses from my colleague @DaveF1006 , which I generally agree with.
By the way , if you are satisfied with answers herein, please consider accepting as solution ( or tell me what more is needed ) so this long thread will close--please.
Does this help ? Is there more I can do for you ?
pk
