An oil rig worker attacked the communications boss at Calico Homes, when he went in the staff entrance and was asked who he was, a court heard.
Thomas Baxter, said to have smelled of drink, spat at Darryl Baker and allegedly punched him in the stomach, after hurling a barrage of verbal abuse at him.
The "sustained" and "degrading" assault took place at the housing association's premises on Croft Street, Burnley.
The town's magistrates were told how 24-year-old Baxter, who earns £40,000 a year, has just returned from Israel.
His solicitor said he wouldn't be in the country to do a community order and asked that he be given a financial penalty instead.
The self-employed defendant was fined £760 - the amount he earns weekly -and was ordered to pay a £76 victim surcharge and £85 costs.
He must also pay £75 compensation to the victim for the distress he caused him.
Miss Catherine Allan, prosecuting, said Mr Baker, head of communications, was leaving Calico by the main staff entrance at 3.30pm, when he saw an unknown man, the defendant, leaning on the door. The victim thought something was not quite right as people did not usually stand there.
The defendant, who smelled of alcohol, entered the building and Mr Baker asked: "Who are you?
"Where are you going?"
Baxter, who said he was there to see Burnley Council, where his mum worked, refused to show any ID and to leave and began to loiter in the corridor. Mr Baker told him he would call the police and the defendant replied:"Go on then."
Miss Allan said Mr Baker was assaulted as he phoned 999. He had guided the defendant down by his coat after he wandered off and went up some steps. Baxter swore at him, demanding: " What do you think you are doing?
"Who are you ?"
The defendant then left towards Burnley station.
Miss Allan added that the victim told police he thought he Mr Baker was a decent person and he worked helping others.
The prosecutor continued: Mr Baker said 'I find it upsetting that somebody who is trying to be a good person can be abused in this way. I am feeling unsafe about coming to work.'
Baxter was identified after his spittle was swabbed for DNA. He had no previous convictions, but had a caution.
Miss Cathryn Fell, defending, said Baxter fully accepted what he did was wrong. She continued: " He says as it was snowing on that day and he went in to get warm. "
Baxter accepted he spat at Mr Baker after an " altercation " with him, but he didn't accept he had punched him.
Miss Fell added: " He's very sorry for what happened."
The defendant, of Gannow Lane, Burnley, admitted assault by beating on Wednesday, February 28th.