When pestered whether Trump's donation would come from the Trump Foundation, the Trump Organisation or from Trump's personal funds, Sanders answered she "would assume that comes directly from him".
President Trump is pledging to chip in to assist Hurricane Harvey recovery efforts, to the tune of $1 million of his personal funds.
She said the President hadn't determined which organization to support.
U.S. President Donald Trump (C) and first lady Melania Trump (R) receive a briefing on Tropical Storm Harvey relief efforts with Texas Governor Greg Abbott (2ndL) in Corpus Christi, Texas on August 29, 2017.
Ms Sanders said he is likely to visit Houston - which was paralysed by the storm earlier this week - and Lake Charles, Louisiana, where Harvey headed after dumping record rainfall on Texas.
Trump calls again on GOP senators to eliminate filibuster rule
That could hurt the United States' credit rating, cause financial turmoil, harm the USA economy and possibly trigger a recession. The warning about a possible US downgrade seemed less dire and narrower in scope than one issued on Wednesday by Fitch Ratings.
"He met with a number of state and local officials who are eating, sleeping, breathing the Harvey disaster", she said in defense of the president.
Trump used a similar "reporters decide" gimmick at the beginning of his administration, that time with his presidential salary, which he is declining to accept. Then-press secretary Sean Spicer told reporters at a March briefing that Trump "has kindly asked you all determine where that goes".
Mr Trump, a former real estate magnate, has been blasted in the past for giving far less of his income to charitable causes than other multi-billionaires.
Harvey's flood waters have heavily damaged tens of thousands of homes across Texas and killed at least 30 people.
In Beaumont, Texas, doctors and nurses evacuated some 190 people from a hospital that halted operations after the storm knocked out water service in the city of nearly 120,000 people.