The House Post Office and Civil Service Committee yesterday approved, 12 to 11, an amendment that would make it easier to fire federal workers.

But the committee may defeat the amendment on a second vote next week, sources said, because of a misunderstanding over the use of a proxy in yesterday's vote.

The sometimes angry debate on the amendment aired some of the committee's fundamental divisions over President Carter's proposals to overhaul the civil service system.

The amendment, offered by committee vice chairman Morris Udall and supported by the Carter forces as a reasonable compromise, would lower the standards of evidence that a federal manager must have in order to fire an employe and make it stick through the appeals process.

Most of the committee's Democrats opposed the amendment. They were led by those with large federal employe constituencies or strong ties to federal employes' unions. Udall said just before the vote that he expected to lose it.

After the vote, a question arose as to whether the proxy of Rep. Michael Myers (D-Pa.) had been miscast as an "aye." He reportedly had instructed his staff to cast his vote "with the unions," in which case the amendment would have been defeated. Myers was out of town and could not be reached for comment. Udall said the committee would be given a chance to reconsider its vote in light of the question raised about Myers' proxy.

The debate revolved around legal distinction between such terms as "reasonable," "Substantial," and "preponderant," as standards of proof.

But the more fundamental "threshold question," Udall said, was whether "your highest goal is to see the least possible injustice is done to federal employes," or to provide "more efficiency in the federal service."

Rep. William Ford (D-Mich.), a leading committee ally of federal employe unions, contended that the amendment would deprive employes of their "property rights" to their federal jobs - a right that Udall argued does not exist.

The amendment would require employers to meet two standards of proof: to fire employes for misconduct, such as drunkenness on the job, they would need a "preponderance of the evidence," and to fire employes for incompetence, they would need a lesser degree of evidence, described as "substantial."

The compromise represents a "considerable retreat" by the administration from its original proposal, Udall said, which would have shifted to employes the burden of proving their firings had been "arbitrary and capricious."