Since this payment ( bonus ) was for work performed in India and before you entered the USA, so even though it is paid in 2018, it would still fall outside your W-2 reporting and hence not taxed by the USA ------ (a) it is for work performed during a period not under US purview; (b) since it is a fixed percentage of your monthly salary, it is really a delayed payment of salary. Suggest your W-2 takes precedence.
IRS does take the position that ( for US taxpayers ) if you receive delayed payment for earlier work, then you report the amount in the year received. Thus if your wages for the year 2017 , earned abroad, but received in 2018, be reported as taxable income in 2018. And generally , if this income is doubly taxed, is eligible for foreign tax credit --- but in your case, I do not believe it will make you whole. Furthermore your US employer has decided that this is not reportable income. Therefore I would leave it alone and use the W-2 as the basis.
Namaste ji
