@xiaoxinwu , I am not fully sure that I am understanding the situation that you are alluding to . So I will first mention my understanding and then go from there
(a) YOU and US citizen/Resident ( Green Card) are living and working/earning abroad
(b) You have some US income
(c) You have foreign tax credit from prior years that may be available for 2020
Let us use a fictional figure for your foreign earnings --- US$ 150,000. You also have US sourced income of $20,000. Your total world income is thus US$170,000. Your host country taxes you on US$150,000, while US taxes you on your world income of US$170,000. You also have foreign tax credit available of US$10,000
(1) Because you have been abroad long enough to satisfy physical presence test, you choose to exclude 2020 max limit of US$107,600, thus reducing your US tax base to US$ 62,400. Note that this means you still have foreign earnings of US$42,400 and foreign tax thereon ( allocated). Assuming further that the foreign country taxed you US$45,000. Thus your total foreign tax credit available for the use is $10,000 + $18720 ( the allocated foreign tax on un-excluded foreign income). So your form 1116 will use the ratio of foreign income ( tax thereon ) to world income ( tax thereon ) i.e. ratio of the two tax categories to compute the allowable foreign tax credit for the year.
2. You could also choose tax deduction ( although these days it is often not a good deal because of SALT limitations ).
3. You could also choose not to use foreign earnings exclusion and claim the whole foreign income for purposes of getting foreign tax credit ---- although this choice once made kinds of sticks with you.
Does this make sense or did I mis-understand your situation ?
