 -- President Obama finally mailed a donation by check to the family foundation set up to further the humanitarian goals of slain American hostage Kayla Mueller, after ABC News reported that for more than a year a private presidential promise went unfulfilled.

The Obamas' check for the Kayla's Hands Foundation arrived from the first family's Chicago residence shortly after the segment, "The Girl Left Behind," aired on ABC News "20/20" in August. It was soon followed by a personal note handwritten by the President on White House stationary apologizing for the 18-month delay in keeping his word, according to Kayla's parents.

"He thought it had already been taken care of," Kayla's mother, Marsha Mueller, told ABC News.

"He basically said it was an oversight," Carl Mueller said.

The couple declined to reveal how much the president gave, saying to disclose what any donor contributed would be inappropriate. "I think he does care about Kayla," her mother said, adding that they are "thankful" for his support.

The Muellers on "20/20" had described meeting privately with Obama in Phoenix a month after the U.S. confirmed their daughter had been killed while held by ISIS in February 2015.

In the interview Carl Mueller, overwhelmed with pain and grief, said, "I’m still waiting for that donation, Mr. President."

A White House spokesperson in August scrambled to say that the President still intended to make it and that Kayla's Hands was the kind of charity the first couple strongly supports -- a rare public endorsement of a charity by the Obamas.

As a result of the "20/20" segment, which told the agonizing story of young idealist Kayla trying to help Syrian war refugees and being kidnapped in Syria in August 2013, over 800 letters of support were received by the Muellers, they said.

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The painful "oversight" by the President was another reminder to the Muellers of the many missteps by the U.S. government, which had failed to save Kayla, they said, after aid groups had abandoned her, an attempted military rescue just missed her and the FBI botched an opportunity to help the Muellers pay ISIS a ransom. Officials said even though U.S. policy does not completely bar such payments, the families of the ISIS hostages in Syria were threatened with being prosecuted for "material support to terrorism" if they did.

That shocking threat from Obama administration officials -- first disclosed by the parents of be-headed journalist James Foley in an interview on "World News Tonight" in September 2014 -- led to the first government review of United States hostage policy in decades and then over the past year a major policy overhaul, which included a promise to never again threaten families who pay ransom for their loved ones, or negotiate with terrorists over a payment, with prison time.

The Muellers also said that the U.S. State Department and the FBI's new Hostage Recovery Fusion Cell offered to pony up a $5 million public reward last year for tips leading to the capture of ISIS terrorists responsible for Kayla's death -- which her parents rejected as a tone-deaf proposal from a government that had blocked every effort they made to pay ISIS a ransom while she was still alive.

"It just doesn't make sense. Typical government logic," Carl Mueller said.

Over the weekend, he accepted an invitation to speak at a campaign rally for Republican presidential candidate Donald Trump in Phoenix. He told his daughter's story and repeated comments he has made in interviews about being "blocked" by the Obama administration in the family's efforts to save her.

"It's time for a change," he told the crowd before the GOP nominee took the stage. "We cannot afford another four years of the same failed policies."

The Muellers said the nonpartisan Kayla's Hands Foundation has received more than 750 individual donations ranging between $1 and $5,000 and totaling $55,000 from viewers of the two-year ABC News investigation featured on "20/20."

"We feel his [Obama's] endorsement of our foundation was helpful in getting some of those donations," said Carl Mueller.

The family is finalizing plans to make a significant donation of almost all of the $130,000 they've raised since Kayla's death to go toward humanitarian work which alleviates suffering and heals emotional wounds.

Months before her reported death, the White House and FBI learned that Kayla was tortured, verbally abused and repeatedly raped by the top leader of ISIS, Abu Bakr al-Baghdadi. But former hostages told ABC News that she had often put the welfare of her fellow captives before her own and had even stood up to ISIS executioner "Jihadi John" to correct him when he told other western hostages she had converted from Christianity to Islam.

The story of their daughter's sacrifice as told by the Muellers has inspired people around the world and was a highlight of a papal conference on religious persecution earlier this year held at the United Nations.

One group of 70 middle schoolers in a special education class in Utah watched excerpts of the ABC News program this month and sent letters to the Mueller family praising Kayla's humanity, with some clipping a dollar bill to their letters, totaling $23, Marsha Mueller said.

The central mission of the foundation is summed up in a new motto the Muellers have adopted, which they say Kayla would have loved: "Human Kindness -- Pass It On."