Paolo Mannici hired the 59-year-old victim to clear his Ravenhall worksite in 2020, paying him an initial deposit of $15,000 with $12,500 still owing.
The invoice stated there would be extra fees if any contaminated soil was found at the site, so when the victim found burnt tyres buried in the soil, he told Mannici he would stop work unless he paid more.
Mannici refused, telling the victim he was broke and could not afford it.
Over the following months, the victim hired debt collectors and there were altercations between Mannici and the man over the disputed money.
The victim dumped rubbish at the site on December 7 and the then-89-year-old went to police twice to report the victim for abuse.
On December 9, Mannici was at his Ravenhall lot when the victim arrived and began dumping another truck load of rubbish.
CCTV footage showed Mannici holding a walking stick in one hand and a hand gun in the other as he approached the victim.
A struggle ensued between the pair and two gun shots went off, one striking the victim in his forearm.
The man required surgery to repair his damaged forearm and he still experiences nerve pain and post-traumatic stress disorder.
Mannici told police he didn't mean to shoot the victim and he only had the gun to protect himself.
The 90-year-old, who faced court on Wednesday, pleaded guilty to engaging in reckless conduct endangering life and possessing an unregistered handgun.
His defence counsel Theo Alexander said the offending was out of character for the Keilor East man, who had no prior convictions.
Dr Alexander told the County Court his client was remorseful and should be spared jail time because of his age and prior good record.
But Judge Damian Murphy said Mannici was acting like a "vigilante" on the day of the shooting and the court needed to deter others from acting similarly.
Judge Murphy extended Mannici's bail ahead of his sentencing on Wednesday afternoon.
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