Christopher had apparently claimed in the papers that he, not Spears' ex-husband Kevin, was the "true father" of the couple's oldest son, Sean Preston. But sources have said the documents appear to have been doctored, reported Us magazine.
The story "does not make any sense," one insider said of the allegations against Spears, noting that the real Christopher is currently working in LA, even though the papers were filed in Florida.
"Chris wouldn't resort to trying to get money like this. Even if it were true. He has a good job, despite the document that says he's unemployed," said a second source.
Still, insiders said that the Federline family is "very concerned" about the case. "The signatures match so well," a source said, pointing out that the person who filed the papers knew details about Christopher's ex-girlfriend and work history. "The family is not done investigating!"