Rep. Charles C. Diggs Jr. (D-Mich) testified yesterday that he did not consider the payment of his personal bills by an employe to be either a gift or a loan but "just . . . her willingness to help me out.

Diggs, on trial in U.S. District Court here on charges of mail fraud and illegallly diverting more than $65,000 in congressional pay to his personal use was the last witness to be heard as testimony concluded.The case will be presented to the jury today following final arguments by prosecution and defense attorneys.

The last evidence presented to the jury was an advisory opinion issued in July 1973 by the House Ethics Committee that goes to the heart of the prosecution's case against Diggs.

The opinion states "that it is improper to levy, as a condition of employment, any responsibility on any clerk to incur personal expenditures for the primary benefit of the member or of the member's congressional office operations, such as subscription to publications or purchase of services, goods or products intended for other than the clerk's own personal use.

Diggs, in his own testimony, has acknowledge that his congressional employees used portions of their salaries to pay "his personal and congressional bills. He asserted, in contradiction to the testimony of at least one former employe, that payment of his employes was entirely voluntary.

Jean G. Stultz, Digg's former office manager, testified last week that he made her payment of his bills virtually "a condition of employment."

Diggs, on his second day of cross-examination yesterday, said he never considered the money from Stultz to be a loan. Asked by Justice Department prosecutor John T. Kotelly if he considered the money to be a gift, Diggs replied, "Mrs Stultz is the one to ask that question."

Kotelly they asked Diggs, who has voted on tax legislation as a congressman, if he had ever advised Stultz that she would have to pay a gift tax on any amount she gave him over $3,000. "That subject never came up during conversations," Diggs replied emphatically. "We didn't talk about gifts or loans or anything like that," Diggs said "I just considered it her willingness to help me out."

Kotelly presented Diggs with wage and tax statements showing that he and his wife made on the average more than $70,000 in gross income each year from 1973 to 1976-the perios covered by the indictment. Diggs repeatedly tried to state what his income after taxes was, asserting at one point that "gross income is grossly misleading."

Diggs' second day of testimony was marked by increased tension as Diggs repeatedly resisted Kotelly's attempt to have Diggs acknowlege that he raised his employes' pay to have them pay bills. "I authorized the salary (increase) forms for Mrs. Stultz and she paid the bills. Those are two separate propositions," Diggs said.

Kotelly asked Diggs if he denied she paid his bills. "I am not denying that," Diggs replied. "I say positively that Jean Stultz paid bills out of her salary."

Kotelly began to ask Diggs, "If yoy're not denying that Jean Stultz paid bills out of her salary, then you are admitting . . ."but he was cut off by U.S. District Court Judge Oliver Gasch, who instructed Kotelly not to argue with the witness.

Later, when Kotelly described Stultz's salary as having been "inflated" to allow her to pay Diggs' bills, Diggs replied, "the word inflation I do not accept. . .I do no accept the terminology."

The trial resumes this morning.