Welsh, 48, was fatally shot at in the face his flat in the West End of Edinburgh on April 17 2019.
Sean Orman, 30, has pleaded not guilty to all 15 charges against him, including murder, attempted murder, firearms and drugs offences, and is on trial at the High Court in Edinburgh.
Dean White told the court in evidence on Thursday that he had seen the accused at his brother Robert's home in Duddingston Row with a man known as Peem in March 2019. He said they were speaking about carrying out an attack on a man and his son in the Oxgangs area for money.
He said the accused then claimed he was due to murder Welsh, who ran a boxing gym, for £10,000, which prompted the witness to alert Police Scotland around March 18.
Mr White said: "He brought a wad of cash out and it was like thousands and he had told us that he had seriously assaulted someone in Oxgangs and the guy that he'd assaulted got in the way and that he seriously assaulted his son as well."
The witness then told the court Orman had claimed to have been paid by Dode Baigrie to carry out the attack.
Mr White added: "“He said his next hit was.. he said he was getting £10,000 to kill Bradley Welsh and I went to the police and I reported this before it happened.
“They didn’t say how but they knew where he lived. They didn’t say how they were going to do it.”
The 49-year-old added he had seen Orman at his brother's property on several subsequent occasions.
He said the accused had turned up there with an "old-styled" shotgun which he was showing off and misfired into the floor of the property.
Mr White said he then told officers about the incident and that they may be able to recover a bullet.
Richard Goddard QC, advocate depute, asked how he had felt about the events in his brother's property.
Mr White said: "Extremely nervous and I went straight to the police station and told them exactly what happened in my brother's house.
He added: “They told me they had a gun and he told me he had a machete as well.”
Mr White said that Mr Orman told him that Peem was his driver and that they destroyed the car which they used to go to the man’s house.
Mr White said: “They had to get rid of the car and the best way to do it was to burn it out so they couldn’t get any evidence.”
Mr White also said that Mr Orman showed him a shotgun which he produced from a holdall. He said that Mr Orman cocked the weapon and it went off.
He said: “He was showing off. He cocked it and it went off in my brother’s house and the bullet went through the floorboards and I told the police they could retrieve his bullets.”
When asked how Mr Orman looked when the weapon was discharged, Mr White said: “It was like ‘Oh my God. I didn’t know it was going to happen.”
Mr White said he felt “extremely nervous” from Mr Orman’s admissions and that he contacted the police.
He said: “I went straight to the police station and told them exactly what was going on. I told them his plan was he was going to get Bradley Welsh and this was a month before the guy got murdered.”
Mr Goddard read the jury a statement of evidence which has been agreed between prosecutors and defence lawyers.
In the statement, Mr Goddard said medics carried out a post mortem of Mr Welsh on April 19 2019 - two days after he was found dead.
He said the cause of Mr Welsh’s death was a shotgun wound to his face.
Mr Goddard added: “The most significant aspect of the shotgun wound was that it had cut the right external carotid artery. This is a major blood vessel on the head and neck. The mechanism of death was blood loss associated with this wound.
“Death may not have been immediate and the deceased my have been capable of some degree of physical activity until the degree of blood loss became critical.
“Pathological findings are that the shotgun was discharged at a distance of approximately one metre from the deceased.”
Mr Goddard also described the moment police firearms officers arrived at Mr Welsh’s home on Chester Street, Edinburgh, at 8.24pm on April 17 2019.
He added: “Police observed the deceased Bradley Welsh lying on his back on the steps. His head was towards the bottom of the steps and his feet were on the stone landing.
“He was seen to have a severe head injury apparently incompatible with life.
“One officer stepped over the body of the deceased and checked for signs of life.
“There were none.
“Four firearms officers then passed the deceased on the stairs to attend at the door of 3a Chester Street with a view to carrying out an emergency search to ensure there were no other injured parties within.
“The deceased’s partner and child were the only occupants and were unaware of the shooting.”
Mr Goddard said that Mr Welsh’s home was also searched under warrant on April 29 2019.
He added: “From within the house were recovered a Nokia mobile phone, an iPhone, three iPads, 2 laptops, a Sim Card, covert body armour, a baton weapon, numerous USB sticks and SD cards, a quantity of old electronic devices, including mobile phones and a Mercedes car key.”
Mr Goddard also said that police searched a Fiat Panda car belonging to Mr Welsh and recovered a knife “from the front passenger side”.
The court has previously heard of alleged attacks on David McMillan, 50, and his son David, 25, at their home at Pitcairn Grove, in Greenbank, Edinburgh, on March 13.
Orman is also accused of driving at speeds up to 123mph on the Edinburgh bypass on April 22 2019 - where the limit is 70mph - and failing to stop for uniformed police officers.
He faces an accusation of having driven on the opposing side of the carriageway of Clovenstone Road "in excess of 92mph", where the limit is 30mph.
Another charge alleges he drove without insurance and was in possession of the class A drug diamorphine and class C drug diazepam.
The trial, before judge Lord Beckett, continues.