"What if she changes her mind or is medically unsuitable?" said Caplan. "We don't know yet if she is physically a good match. HIV, hepatitis or cancer could knock her out."
"For more than 50 years in the U.S. and in law and ethics, quid pro quos are not acceptable. Her sister should have the right to change her mind until they wheel her into the hospital and take out the kidney," he said.
But the governor told the Associated Press said the issue doesn't worry him.
"All of the 'What if' questions are, at this point, purely hypothetical," said Barbour said in a prepared statement. "We'll deal with those situations if they actually happen."
The Scott sisters, who are African American and had no prior record, have been backed by civil rights advocates who hailed the governor's decision, saying it corrected a "miscarriage of justice."
They have insisted they were innocent and that witnesses were pressured into testifying against them. Three boys, who netted only $11 in the robbery, served only two years of an eight-year sentence.
The Scott sisters, who had five children between, served 16 years in prison and had exhausted all appeals. They would have been eligible for parole in 2014.
Barbour had intervened twice in the case, asking the parole board to give the Scott sisters early release so Jamie Scott could get a transplant, according to his press secretary Daniel Turner.
"There was a medical necessity, as a preferential consideration," said Turner. "It's also expensive for the taxpayers to keep a seriously ill person incarcerated. If [Jamie] can have this procedure, it very likely will be done through Medicaid.There are a lot of facets to it and there are arguments each side of the ledger."
Some ethicists say the governor should have separated the issues of early release and organ donation.
"He could recognize her generous offer and take that into account in determining whether the prisoner was worthy of parole or release. In society, we usually try to determine how the character has changed or evolved while in prison," said Felicia Cohn, bioethical director for Kaiser Permanente in Orange County, Calif. "If she made a generous gift to her sister that might qualify for release, but it shouldn't be a condition for release."
Cohn recognizes that society wants to encourage altruism, but said, "I am not sure this fits the definition."
"Altruism means behaving selflessly and giving for the benefit of others and some argue that we can never be fully altruistic," she said. 'Most of us derive some personal benefit or pleasure for giving to others. But the idea is that we don't benefit in any way."
Gladys Scott has said she wants to donate a kidney to her ailing sister, but Cohn said, "I wouldn't describe it as a gift. Essentially one sister is being paid for her kidney. It's not monetary payment, but it's her freedom, which is worth even more. Our freedom is considered invaluable."