Claudia Rendon, a 41-year-old Philadelphia mother, says she was fired from her job in a school admissions office after she took time off to donate a kidney to her son.

Rendon's employer, Aviation Institute of Maintenance, put her back on salary Tuesday after a media firestorm.

But the incident has thrown holes in the Family and Medical Leave Act into the spotlight; her employer has less than 50 employees, and therefore is not required to provide the 12 weeks of unpaid leave the act requires. She could, however, have had a case on the Americans With Disabilities Act.

"The issue is whether her surgery and complications would constitute a physical impairment substantially limiting a major life activity. That is basically the legal definition from these laws," Michael Foreman, a clinical law professor and director of Penn State's Civil Rights Appellate Clinic, told ABC News.

If it's determined that it does, the employer would have to provide "reasonable accommodation" requiring an examination of how keeping the position open could harm the company's business.

Here's what happened: Alex, Rendon's 22-year-old son (who's also a student at the Aviation Institute), suffered a kidney failure last January. Rendon was found to be a match.

So Rendon, who'd worked at the school for a year and a half, told her bosses she'd be having transplant surgery on July 21 and planned to take leave starting July 19. It typically takes six to eight weeks to recover from kidney surgery, and Rendon says her bosses granted her unpaid leave until September 1.

On her last day of work before the operation, her manager promised her that her job was safe, she says, but an hour later, asked her to sign a letter acknowledging that it was not.

"They said, 'If you don't sign this letter, you are abandoning your job and quitting,'" Rendon told ABCNews. "I said, 'I am not abandoning my job. I am saving my son's life.'"

She signed the letter after being told she was a "good employee" and her job would probably be there when she got back. Rendon had used her vacation time earlier in the year because of family illness, including a week in Colombia to bury her mother, who died in February.

On August 24, Rendon called the school to say she might not be able to return September 1 because of back pain. The school asked for a letter from the hospital, which she provided, along with a letter from her short-term disability provider. Both said she would return to work September 12.

On September 8 Rendon, who said she could walk only about 15 minutes without aid because of the pain, popped into the office for a social visit. That's when she found out that, because of what the school called business needs, her job had been filled two days before. She was shocked.

"If they would have told me to come back that day, I would have done it," Rendon said. (The school also is chasing her son for $2,000 related to the time he took off for medical reasons. It also wants a $150 re-enrollment fee.)

Philadelphia's Fox 29 on Monday asked Rendon's boss, Kyle Berry, why the company filled her job. He had no comment and asked the TV reporters to leave. The institute told the TV station her firing was within the company's legal rights.

The next day, Tuesday, the company put Rendon back on full salary until another position opens at the company. Berry's reason for this: "We had time to reconsider. It was simply a mistake."

The pay isn't forever: When a position opens up, Rendon can reapply for it, but the company is not promising to give her her job back.

Rendon is skeptical and has hired a lawyer.