@joysyt , while generally agreeing with @ThomasM125 , I would like to add the following
(a) since the actual work was done in Singapore , for US tax purposes this is foreign income and therefore eligible for Foreign Earned income exclusion or foreign Tax Credit / Deduction. Your US employer was in error , treating you as local employee. However the good news out of that is 1. your normal federal taxes have been withheld at source, your FICA taxes have been withheld/paid.
(b) Since Singapore tax year is calendar year, the amount of tax due to Singapore as a resident was hopefully paid and if not then file the Singapore return first so you have a proper figure of the assessment. Then use this as the taxes paid to a foreign taxing authority.
(c) When completing form 1116 ( as described by @ThomasM125 ) declare the income while in Singapore as your foreign income and foreign taxes paid as paid. Note that while the foreign taxes paid is recognized dollar for dollar, actual amount allowable for the tax year is based on a foreign income to world income and thus can be a bit less than the actual amount of foreign taxes paid ( depending on the exact facts and circumstances ). The disallowed portion of the credit may be carried back or forward as long as there is foreign income in those years. Thus it tends to be , at best, asymptotic. You can also deduct the taxes ( like states taxes ) if you itemize but again you have SALT ( State And Local Taxes ) limit of 10KUS$.
(d) Note that if the final taxes paid to foreign taxation authority is different than what you claim on form 1040/ form m1116, then once the dust settles, you may have to file an amended return to IRS / State recognizing this.
(e) Note that Singapore does not have a totalization agreement with the USA and thus you may have to pay Social Security / Medicare equivalent taxes to both country.
Does the above make sense ? Is there more I can do for you ?
