Jake Walker, 25, of West Road, Lancaster, had only been released from a robbery sentence a few weeks before the attack, which left his victim with a 5cm cut to his lip, a 1cm cut to his face and a 1cm cut to his bicep.
Prosecuting, Gwen Henshaw said Walker, his victim and another man who witnessed what happened had all been living in Oak Tree House - a hostel type accommodation for ex offenders, homeless people and those with drug addiction issues.
Victim Gary Dean and another man, James Newbury, were outside the property at 2.30pm on January 12, 2019, when Walker leaned out of a window and threw down a bag of change, asking them to "go to shop and change it for a £20."
Miss Henshaw added: " Mr Dean counted the change and found there was only £18 and phoned the defendant to let him know. That didn't, at that stage, cause any problems.
"They changed some of the money for a £10 note and walked back."
When they arrived back there was a man waiting under the window and Walker shouted down to tell them to give him the £10 for a bag of marijuana.
The pair went inside and handed the drugs and change to Walker.
He then asked Mr Newbury to go and "buy some skunk" with £5.
However, Mr Newbury returned with a bag of heroin which led to an argument.
Miss Henshaw added: " The defendant began pushing Garry Dean, who returned to James Newbury's room.
"The defendant followed him saying: 'I'm not having this, I'd kill a man for this.'
"Mr Dean saw a red and black Stanley knife in his right hand as the defendant started to move towards him.
"Garry Dean grabbed his upper body to try to fend him off and pushed him towards the bed in self defence.
"He slashed his face causing lacerations to his lower lip and right eye and also a cut to his bicep."
Staff were alerted and contacted the emergency services, and Mr Dean was taken by ambulance to the Royal LAncaster Infirmary, where his wounds were sutured by maxillofacial surgeons.
In a victim impact statement he described suffering significant emotional difficulties since the offence including "trust issues, stress and panic attacks" and was self conscious about the scarring.
Police launched a search for Walker and shortly after midnight two officers located a vehicle he was suspected to be travelling in and he was seen lying across the rear seat.
He was recalled to prison on licence for robbery.
Recorder Michael Maher reduced his jail sentence to 18 months on account of current prison conditions, in which inmates are spending 23 hours a day in their cell with no access to courses, and Walker's mum's cancer diagnosis.
However he said the fact he was on licence for robbery aggravated the offence.
He said: " The victim saw you had a Stanley knife in your possession,
"Mr Dean tried to defend himself but as photographs show all too well he was singularly unsuccessful in that regard.
"He remains self conscious about the scars, notwithstanding the skill and talents of the surgeons."
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