Melania Trump appears on stage after U.S. Republican presidential nominee Donald Trump speech at the Republican National Convention in Cleveland, Ohio, U.S. July 21, 2016.

Melania Trump appears on stage after U.S. Republican presidential nominee Donald Trump speech at the Republican National Convention in Cleveland, Ohio, U.S. July 21, 2016. Reuters/Jim Young

US President Donald Trump allegedly lied about the engagement ring he purchased for wife Melania Trump. The issue concerns a discount he received for a 10-carat Graff Diamonds ring in 2005.

Trump told the New York Times in 2015 that Graff Diamonds gave him a US$1 million (AU$1.29 million) discount on her 15-carat engagement ring. For a story about Trump’s wedding with Melania, he reportedly said “only a fool” would pay “a million dollars more for a diamond.”

But a new report from Forbes suggests that the US president's claims were false. Graff Diamonds' chairman Laurence Graff, from his home in Gstaad, Switzerland, said it was “a pleasure to do business” with Trump.

But Graff revealed that “no favours” were given when Trump purchased the D-flawless cut 10-carat diamond. Another source said to have knowledge about the purchase supported Graff’s claim, saying that it was paid in full immediately. Trump’s camp has not commented on the report yet.

As a result of the discount Trump claimed to have received, the London-based company was said to have acquired increased publicity. But Graff CEO Nicholas Paine said they do not sell items for publicity value. Graff and Paine did not disclose a price with Forbes.

When Trump and his wife celebrated their 10th year anniversary in 2015, he allegedly purchased another diamond for Melania. The first lady posed with a large 25-carat ring for her official White House picture. The ring reportedly costs millions.

Official Portrait of First Lady pic.twitter.com/K1DUVE5kSI — Melania Trump (@FLOTUS) April 3, 2017

The FLOTUS, born Melanija Knavs in 1970, was from a Slovenian town. She pursued her dreams of becoming a supermodel and eventually married Trump. She moved to New York in 1996 after working in Milan and Paris. She and Trump met two years after she moved to NYC.

Melania has previously shared how she met her husband, saying it was at a fashion party in September 1998 in New York. “I liked him, he had great charm, but he arrived with a date,” she told Greta Van Susteren during a July 2016 interview for “On the Record.” But still, she said they had a connection from the beginning and described Trump as someone who was “very charming with a great mind and great energy.”

"I went crazy when I saw her,” Trump said in 2005. There is a 24-year age gap between the first couple. She is 46 and he is 70.