The condition is alarming some experts, who have raised legal and ethical questions. Among them: If it turns out the sisters aren't a good tissue match, does that mean the healthy one goes back to jail?
Gov. Haley Barbour's decision to suspend the life sentences of Jamie and Gladys Scott was applauded by civil rights organizations and the women's attorney, who have long said the sentences were too harsh for the crime.
The sisters are black, and their case has been a cause celebre in the state's African-American community.
The Scotts were convicted in 1994 of leading two men into an ambush in central Mississippi the year before. Three teenagers hit each man in the head with a shotgun and took their wallets - making off with only $11, court records said.
After 16 years in prison, Jamie Scott, 36, is on daily dialysis, which officials say costs the state about $200,000 a year.
Barbour agreed to release her because of her medical condition, but 38-year-old Gladys Scott's release order says one of the conditions she must meet is to donate the kidney within one year.
The idea to donate the kidney was Gladys Scott's and she volunteered to do it in her petition for early release.
National NAACP President and CEO Benjamin Todd Jealous thanked Barbour on Thursday after meeting him at the state Capitol in Jackson, calling his decision "a shining example" of the way a governor should use the power of clemency.
Others aren't so sure.
Arthur Caplan, the director of the Center for Bioethics at the University of Pennsylvania, has studied transplants and their legal and ethical ramifications for about 25 years. He said he's never heard of anything like this.
Even though Gladys Scott proposed the idea in her petition for an early release and volunteered to donate the organ, Caplan said, it is against the law to buy and sell organs or to force people to give one up.
"When you volunteer to give a kidney, you're usually free and clear to change your mind right up to the last minute," he said. "When you put a condition on it that you could go back to prison, that's a pretty powerful incentive."
So what happens if she decides, minutes from surgery, to back off the donation?
"My understanding is that she's committed to doing this. This is something that she came up with," said Barbour's spokesman, Dan Turner. "This is not an idea the governor's office brokered. It's not a quid pro quo."
What happens if medical testing determines that the two are not compatible for a transplant? Turner said the sisters are a blood-type match, but that tests to determine tissue compatibility still need to be done.
If they don't match, or if she backs out, will she be heading back to prison?
"All of the 'What if' questions are, at this point, purely hypothetical," Barbour said in a statement from his office late Thursday. "We'll deal with those situations if they actually happen."
Legally, there should be no problems since Gladys Scott volunteered to donate the kidney, said George Cochran, a professor at the University of Mississippi School of Law who specializes in constitutional matters.
"You have a constitutional right to body integrity, but when you consent (to donate an organ) you waive that" right, he said.
Other experts said the sisters' incarceration and their desire for a transplant operation are two separate matters and should not be tied together.
Dr. Michael Shapiro, chief of organ transplants at Hackensack University Medical Center in New Jersey and the chair of the ethics committee at the United Network for Organ Sharing, said the organ transplant should not be a condition of release.
"The simple answer to that is you can't pay someone for a kidney," Shapiro said. "If the governor is trading someone 20 years for a kidney, that might potentially violate the valuable consideration clause" in federal regulations.
That clause is meant to prohibit the buying or selling of organs, and Shapiro said the Scott sisters' situation could violate that rule because it could be construed as trading a thing of value - freedom from prison - for an organ.
Putting conditions on parole, however, is a long-standing practice. And governors granting clemency have sometimes imposed unusual ones, such as requiring people whose sentences are reduced to move elsewhere.
In 1986, South Dakota Gov. Bill Janklow commuted the sentences of 36 criminals, but only on the condition that they leave his state and never come back. In Florida, the governor and members of his cabinet voted in 1994 to reduce a convicted killer's sentence as long as he agreed to live in Maryland.
Whatever the legal or ethical implications of Barbour's decision, it thrust him back into the spotlight, after his recent comments in a magazine article about growing up in the segregated South struck some as racially insensitive.
In the article, Barbour explained that the public schools in his hometown of Yazoo City didn't see the violence that other towns did, and attributed that to the all-white Citizens Council in Mississippi.
Some critics said he glossed over the group's role in segregation. He later said he wasn't defending the group.
The Scott sisters' attorney, Chokwe Lumumba, said people have asked if Barbour, who is mentioned as a potential presidential contender in 2012, suspended their sentences for political reasons.
"My guess is he did," Lumumba said, but he still said the governor did the right thing.
Mississippi Rep. George Flaggs, an outspoken Democrat in the state legislature and an African-American, scoffed at suggestions that Barbour's motive was political and said the decision wasn't an attempt to gloss over the magazine comments.
Flaggs said Barbour suspended the sentences "not only to let this woman out of prison, but to save her life.
"If she doesn't get a kidney, she's going to die," he said.