McChrystal was sacked about a year after receiving his fourth star -- half the time normally necessary to qualify for a four-star general's retirement income of $12,475 per month, before taxes, according to Pentagon estimates based on his 34 years of service.
It was unclear whether Obama might need to issue a waiver.
McChrystal informed the Army of his planned retirement on Monday, a widely expected move after he and his aides enraged the White House by mocking the president and top civilian advisers in an article in Rolling Stone magazine.
In the piece, McChrystal himself made belittling remarks about Vice President Joe Biden and the U.S. special envoy to Afghanistan and Pakistan, Richard Holbrooke. His aides were quoted calling White House national security adviser Jim Jones a "clown.
"Had Obama not been willing to assist McChrystal, he would have retired on a three star general's salary of $11,736 per month, before taxes, according to Pentagon estimates, taking into consideration McChrystal's time in the military.
Obama named General David Petraeus to replace McChrystal. During his confirmation hearing before a Senate committee on Tuesday, Petraeus played down hopes for a swift turnaround after nine years of war.