D.C. Del. Walter E. Fauntroy said yesterday that he will hire former Michigan Rep. Charles C. Diggs Jr. as a special assistant to the chairman and consultant to the Congressional Black Caucus in an effort to show how society may "extend a helping hand . . . to those who have paid their debt to society."

Diggs, once the ranking black member of the House, founder of the caucus, and former chairman of the House District Committee, was convicted in a $60,000 congressional payroll kickback scheme in 1978 and has served seven months of his three-year prison term. He will work for the caucus and Fauntroy, the present chairman of the group, while he serves out his 90-day stay in a Mount Pleasant halfway house, preparing for full reentry into society.

"Our action is consistent with our view that people who have paid their debt to society should be given an opportunity to be proven worthy of trust," said Fauntroy. He said the caucus vote to name Diggs to the position was 17 to 1.

Fauntroy also said the Diggs appointment was "in much the same spirit" with which the Republicans temporarily hired former Maryland Rep. Robert E. Bauman as a consultant to House Minority Whip Trent Lott (r-Miss.). Buaman lost his seat in November after he admitted problems with homosexuality and alcoholism. Bauman finished his 60-day assignment Wednesday and has now returned to private law practice in Easton, Md.

One thing that will not be similar in the two cases, however, is salary. Fauntroy said that the Caucus "won't be able to pay anywhere near" the $1,000 a week that Bauman received from the Republicans. "We are looking at our budget now," said Fauntroy. "I can assure you it won't be a $1,000 per week."

Diggs, formerly chairman of the House subcommittee on Africa and credited with steering home rule for the District of Columbia through Congress, will work on the alternative budget proposal that the caucus is preparing in response to the Reagan administration budget cuts, particularly in the area of foreign affairs, Fauntroy said. Diggs will also work on a redistricting rpoject concerned with rpeserving or enhancing black representation in states where the number of representatives will be changing because of the 1980 Census.

Fauntroy said discussion to bring Diggs back to the caucus began several months ago when he visited Diggs at the minimum-security prison at Maxwell Air Force Base outside Montgomery, Ala. Diggs, who could not be reached for comment, will work at the caucus during the day, but must return to the house, at 1770 Park Rd. NW, each night. But since he is now employed, his curfew will be extended from 10 p.m. to midnight.

Fauntroy said that the caucus has not hired other ex-offenders. But referring to his other profession, that of Baptist minister, he said, "Of course, I do it all the time. As one experienced in the prison ministry, I know that people make mistakes.But one mistake that society makes is treating convictions as a life sentence of rejection for people . . . In addition to (Diggs) great value to (the caucus) over the next 90 days, it will be an instructive example to those who so often treat such sentences as lifetime rejections."