Tax home and temp work in US

I'm assuming you're working in the US for a US company and will be residing in the US for the 3 months.  Also you state your a tax resident of Brazil. I'm also not a tax professional so take everything herein with a grain of salt.

Under these circumstances you will pay taxes as normal in the US, as if you had no other tax base, and you will need to pay income tax on that income to Brazil as well.  The US federal income tax will offset your Brazil tax obligations.

Tax residents of Brazil pay tax on all their global income, so even though you're working outside Brazil for this time you will still owe tax. As mentioned the US federal income tax (but not SS, Medicare or state taxes) can be used to offset the Brazil income tax, and the deduction is 1:1.

The US Company should deduct your income as normal. In Brazil you will need to file a Carne Leao DARF monthly and pay the DARF up to 30 days following the month in which the income was earned.  For instance, if you begin work in December, in January you can create the DARF for the Carne Leao income tax due and pay it by the last day of Jan. You can make the DARF yourself online or hire a Brazilian tax professional to do it for you.  I suggest the latter unless you're already familiar with the process.  Here's the site where you can make a DARF for Carne Leao:

https://sicalc.receita.economia.gov.br/sicalc/rapido/contribuinte

When filing income tax returns in the US, I do not believe you will qualify for the Foreign Earned Income Exemption or the Foreign Tax Credit as the income would likely not be considered foreign sourced since you we're living in the US while earning the income.

thanks for the reply. The question is about compensation for travel related expenses (housing, transportation). I will be working in the us for a US company.  if you travel within the US from your tax home those compensated expenses are not taxable. i want to know if i lose that benefit if my tax home is Brazil?

Oh sorry; well if your job in the US reimburses you for travel, you'll have to ask them what they'll reimburse you for. It's up to the company to set a travel expense reimbursement policy.

I work for a US company, and the one time I traveled back for a visit to the US office they reimbursed me for my airfare, hotel, rental car, meals and so forth. 

I expense my mobile phone bill in Brazil every month as well to my US company for reimbursement.

Expense reimbursements are not considered income, so there should be no tax ramifications of receiving expense reimbursements in either the US or Brazil.   Expense reimbursements do not show up on your US paystub as income, so if tax authorities in Brazil ever audit you there's no extra taxable income to worry about them discovering.

If you're worried about transferring the money back to Brazil after it's reimbursed, the company should provide a separate statement showing the amount reimbursed, and you can use that receipt as documentation showing the source of funds for the transfer, if such documentation is requested.

One thing to consider is, if traveling in the US, you are better off using a US credit card to pay for things.  Brazil imposes a tax on foreign transactions (IOF), not to mention any foreign transaction fees that may be related to the bank/credit card itself.

Traveling out of Brazil for only 3 months doesn't change your tax status here, and leaving the US for good as a US citizen doesn't change your tax status there either.

Hopefully something I said their helps answer your question, if not please explain the specifics of your concern in a bit more detail.

@BRBC thanks that is helpful! my work scared me by saying if i dont have a tax home it is considered income so speak to a tax professional. That said they hadnt run into the scenario before.