Saturday's canceled wedding of Usher and Tameka Foster – who is due to have the couple's baby this fall – was the result of the past few bumpy days for the couple, a source tells PEOPLE.

At issue were family matters and the event itself.

"Usher's mother [and former manager, Jonetta Patton, whom he fired in May] is against the wedding. That is one of the main obstacles," according to the source, who also said of the bride, 37, and groom, 28: "They only decided two weeks ago to have a wedding. They wanted to have it take place before the baby bump started showing."

But as another source tells PEOPLE: "Usher changed his mind."

Usher (full name: Usher Raymond) and Foster, a stylist, also "had a lot of differences about the details. For example, the bride wanted barbeque, [and] he wanted to have [renowned chef] Jean Georges cook," said the source.

As a result of the differences, the wedding "was on again off again for the past few days. [Foster] was calling it off because she wasn't getting the things that she wanted. The mother was calling it off. ... Usher was calling it off."

On Saturday, only hours before the afternoon ceremony was to take place at the Southampton, N.Y., estate of music mogul L.A. Reid, Usher's publicist Patti Webster issued the following statement: "It was announced today that the wedding ceremony for Usher Raymond IV and Tameka Foster was canceled. No additional information will be given regarding the circumstances of the cancellation, but we hope the privacy of this matter will be respected."

According to the New York Post, the 70 to 100 guests who needed to be informed there was to be no wedding included couples Beyoncé and Jay Z, as well as Janet Jackson and Jermaine Dupri.

The paper also said Usher's godfather, Broadway star Ben Vereen, had been removed from the guest list because of his view toward Foster, and that Usher's mother had not been invited to begin with.

New York's Daily News reports that Usher's mother was invited at the last minute, though an unnamed source tells the paper she did not want to go.

"The tents were up," an insider tells PEOPLE. "And they paid all of the people, the vendors, so that wasn't an issue. ... Hotel rooms were an issue. Janet [Jackson] was coming, but she had eight security people. Ashanti and Nelly were coming."

The source added that it was a possibility that the wedding may still come off. On Saturday, however, Webster would not comment on whether Usher and Foster were still engaged or even remained a couple.

Foster was previously married to Noontime Music cofounder Ryan Glover, with whom she has three children.

Speaking about her relationship with Usher, Foster told Essence.com last month, "He just accepted me for who I am. I have flaws, I have an attitude problem. ... Overall, I have issues that a person could either love or hate, but he accepted me completely. And every day he still treats me like we just met."

Earlier this month, in an open letter to PEOPLE, Usher addressed rumors that he was somehow being coerced into getting married. "I am happy, excited, completely clear and independent on my direction, feelings, decisions," he wrote, "and I am NOT BEING LED."