The Giving Pledge, started by Microsoft founder Gates and US investor Warren Buffett to encourage the extremely wealthy to promise to give half their wealth to good causes, announced this week that software mogul Plattner, one of Germany's richest men, had signed up.
But a disgruntled Plattner later said he had made no such commitment to give away half of his money.
"I feel Bill Gates has gone behind my back and used me," Plattner told regional newspaper the Potsdamer Neueste Nachrichten on Thursday.
The charity had quoted from a letter Plattner had wrote praising the practically free higher education he had received in Germany, and talking about wanting to give something back to society.
He and a handful of other billionaires including British tycoon Richard Branson, had signed up to the pledge, the letter said. A condition of joining is that the new member promises to give away at least half their wealth to good causes, although there is no stipulation of where the money goes.
Plattner, 69, is estimated to be worth more than €5 billion. He started a foundation to promote education and health care around 20 years ago - but has not filled it with half of his fortune, much of which is still tied up in SAP, the software firm he founded.
He said he was furious at The Giving Pledge organizers, telling the Potsdamer Neueste Nachrichten that he had not signed up, would not be bounced into doing so, as he intended to retain control of both his stake of SAP and how he funded good causes.
Yet on Wednesday evening SAP itself released a statement saying Plattner was pleased to have joined the Gates-Buffett foundation - seemingly contradicting him.
Just a few hours later he repeated his denial that he was signing up to the foundation.
"They can all write that I am a member of the foundation as far as I am concerned. But I know that I do not fulfil the criteria - whatever Bill Gates says. There is less than 20 percent of my fortune in my foundation," he said.
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The Potsdamer Neueste Nachrichten quoted a spokesman for The Giving Pledge saying that Gates knew about Plattner's social engagement and considered that this fulfilled the criteria.
"Gates will have to explain the rest," Plattner told the paper.