An official of Giant Food Inc. disputed some key prosecution testimony today at the trial of former Teamster local official Francis C. DeBrouse on charges of labor racketeering, extortion and tax fraud.

The testimony of David B. Sykes, Giant Food senior vice president for finance, came as the trial entered its fourth week and the defense prepared to rest its case. DeBrouse, who headed the 8,000-member Teamster Local 639 until 1977, is charged with having used his union position to obtain more than $200,000 worth of goods and services from companies that hired Teamster labor.

The prosecution has prosented testimony intended to prove that Giant subsidized $19,000 worth of architectural design work for DeBrouse, including the plans for his house in Davidsonville, Md.

The design work was done by Robert N. Picardat, who did design work for Giant until 1972, when he left the grocery chain. Giant continued to pay picardat's salary for 18 months thereafter.

Picardat, testifying for the prosecution, had described the payments as a retainer, under which he continued to do some work for Giant. Designing the DeBrouse house was part of that work, he said.

Today, Sykes conceded under questioning by defense attorney Albert Ahern he had told an FBI agent that Picardat left because he was involved in an affair with a woman "related to a high official at Giant."

The money Picardat received was for "past services" to Gaint, Sykes said under questioning. Ahern had asked Sykes if he considered it a bona fide tax deductible business expense to pay Picardat "in order to keep him away from a girl at Giant."

In other testimony, Sykes said that Giant maintained 1,000 to 1,500 "trade accounts" similar to the account set up for Debrouse by Giant to allow DeBrouse never paid Giant for the carpet, according to testimony.

The defense also called representatives of several companies who employes Local 639 had represented to testify to DeBrouse's toughness as a labor negotiator.