The former office manager for Rep. Charles C. Diggs Jr. (D-Mich.) testified yesterday that, at Diggs' direction, she unwillingly paid from her congressional salary thousands of dollars for his personal expenses, ranging from magazine subscriptions to Diggs' home mortgage payments.

The picture of Diggs that emerged from the testimony of Jean G. Stultz, the former aide, was of a man overwhelmed by debts. Toward the end of her employment with Diggs, Stultz testified, "the accounts had gotten into such a drastic state, I was on the phone constantly with creditors," some of whom were threatening to sue Diggs.

Stultz detailed under oath in U.S. District Court here how Diggs raised her congressional salary and then directed her every month to pay his personal and congressional bills from the difference between her "normal" pay and the surplus she was receiving.

Testifying under an assurance of immunity from government prosecutors, Stultz said that Diggs called her into his office near the end of 1973. She said he told her that "there were certain items to be paid and he would increase my salary" and tell her what bills of his to pay with the increase.

"I objected to it," Stultz testified. "I told the congressman I didn't think it was legal . . ." But, she said, Diggs "said an employe could do whatever he wanted with his salary." Finally, she testified she agreed, "but I didn't want to do it for long."

Diggs, chairman of the House District Committee, appointed Stultz to the committee staff while keeping her on his congressional office staff. As a result, she was paid two salaries. Every month, she testified, Diggs would go over his bills with her, directing her how to spend the money she was being paid as a member of the committee staff. She testified that she "performed no actual duties or specific assignments" for the committee.

Diggs was charged last March with mail fraud and misuse of more than $101,000 in federal funds in connection with an alleged scheme in which he raised the salary of some of his congressional employes and then took kickbacks from him to pay his personal and congressional expenses.

Stultz, a precise and poised witness, identified a series of personal checks that she said she had written to pay Diggs' bills as he had directed. She also identified a series of cashier checks and money orders that she said she had purchased from the Riggs National Bank, using the "overage" from her congressional salary. These checks and money orders also were used to pay Diggs' bills, she testified, and his name was typed in as the purchaser.

According to Stultz, Diggs was getting about $1,200 a month in kickbacks from her congressional salary. She estimated her own take-home pay at about $800 a month.

Stultz testified that she and Diggs referred to the extra pay she received as the "special account" and that she regarded it not as her pay but as "his money." Asked if she could have used more money for herself, she replied, "I had bills like everyone else. I had to work. I needed to work. I had a child to support . . . I had sufficient bills - more than enough."

Among the bills Stultz testified she paid for Diggs were several months of mortgage payments for his home on Capitol Hill, thousands of dollars in personal loans extended by banks and credit companies in Washington, Detroit and elsewhere, life and car insurance premiums, a $51 bill for tuning his car and a 5 subscription to Moneysworth magazine. Stultz also testified that she paid bills for congressional expenses incurred by Diggs for both his Washington and Detroit offices.

"These payments were always made at the direction of Congressman Diggs," she told the jury.

At one point, Diggs suggested that an employe in his Detroit congressional office should receive an increase in order to pay some of Diggs' bills, Stultz testified. She said she told Diggs that it would not be a good idea since the employe was a lawyer and might object. When Diggs asked her whom she would suggest, she testified that she would not suggest anyone but that another employe, Felix Matlock, was the "most loyal" and Diggs told her to "'see what you can work out.'"

Matlock's salary was subsequently increased, she testified, and he called her for instructions about how to spend the extra money. "The actual instructions to Mr. Matlock came from me, but I got those instructions from the congressman," she said.

The use of money orders and cashier's checks was Diggs' idea, Stultz testified. "One thing I remember most clearly," she said, "is that he said he was going to make a politician out of me yet."

Stultz's testimony was interrupted before the prosecutor John Kotelly had completed questioning her when her daughter, Sharon Williams, fainted in the courtroom. U.S. District Court Judge Oliver Gasch cleared the courtroom of spectators and the jury while the courthouse nurse revived Williams. Stultz then accompanied her to a hospital.

Gasch told the jury that "a relative" of Stultz's had become ill and that Stultz would resume testifying today. Two other prosecution witnesses, employes of Detroit banks, testified briefly before Gasch recessed the trial for th day.

The trial resumes this morning.