A spokesman for the top USA diplomat said Tillerson had cut short his first trip to Africa to return to Washington about one day ahead of schedule to deal with urgent work there.
Tillerson, the highest-ranking US official ever to visit Chad, made the comments in the capital, N'Djamena, on the final day of a five-nation African tour.
His meeting with the president was announced by Buhari's Personal Assistant on New Media, Bashir Ahmad on Twitter.
He arrived in Abuja, Nigeria later Monday for the final stop of a trip that included visits to Ethiopia, Djibouti and Kenya.
On his part, Mr Tillerson reaffirmed the support of the US for Nigeria to ensure the speedy recovery of the abducted girls.
Tillerson's job was made even harder in January after Trump allegedly called African nations "shithole" countries.
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"The placement of Chad in this list was an injustice done to Chad", Foreign Minister Mahamat Zene Cherif said.
In the case of the elephants, conservationists appeared to have a powerful ally in President Donald Trump, who intervened personally previous year to stop the U.S. Fish and Wildlife Service from lifting the Obama-era ban on tusks imported from Zambia and Zimbabwe.
Tillerson was speaking in the capital N'Djamena, said a report on Chad's progress toward strengthening controls on passports and improving information sharing was being prepared in Washington. "These steps are going to allow us to take action to begin to normalize" the situation, he said.
The Chad issue has emerged as a sore point between the State Department and Homeland Security, exposing fault lines within Trump's administration.
Trump accepted the invitation Thursday, saying he would meet with North Korean leader Kim Jong-un by May to achieve the permanent denuclearization of the regime, according to the South Korean envoy who served as an intermediary between Washington and Pyongyang.
Tillerson, while in East Africa, cancelled scheduled events on Saturday on the grounds of ill-health.