If you are a US taxpayer (as a citizen, green card holder, or resident alien) then whether or not the payment is taxable depends on US tax law, not the laws of the country where the payment was made.
Unfortunately, I can’t find a clear answer on this, so maybe another expert will have a better answer. On one hand, if the employer has no obligation to make the payment, then it might be considered a gift. On the other hand, if the payment is meant to supplement a pension because of inflation, and especially because the employer is only making similar gifts to former employees, then it sounds like this could be taxable income as payment for prior services or as an increased pension. I found one tax court case that indicated that a similar “gift” was taxable compensation, but the facts are so different that I don’t think that case helps us here. I will be interested to see what other people think.
