The fines date back to 2010 but the man, known as Miguel, was not informed of them until December last year, according to his lawyer, Roberto Castro.
"He has collapsed and plunged into a depression," Castro told Spanish daily 20 Minutos.
He added: "The council didn't send the fines because they didn't have his address, but they had his phone number. His family struggles to make ends meet and so he sought to earn some extra income by doing furniture removals."
"Miguel is unemployed and only receives a widower's pension of less than a thousand euros per month, with which he must keep his two sons, aged 14 and 19, and pay the rent for the flat in which they live," he said.
Unauthorized bill-posting in Barcelona carries an automatic minimum fine of €375, which can go up if there are more than five instances.
Police in nearby Sant Cugat del Vallès, Gavà and Terrassa, where Miguel had also stuck posters, used the phone number on the advertisements to contact him and warn him to stop, which he did.
"They used common sense," said Castro.
"When you fine someone 300 times you know you're plunging them into misery," he added.
"Paying it off in €50 monthly instalments, it would take four generations to clear the debt."
Castro told reporters that he had filed an action claiming multiple procedural errors, and that he had not ruled out asking for help from Barcelona's public defender.
He hoped that the media interest would lead to the punishment being lifted.
Barcelona's City Hall are said to be studying the allegations made by Castro and are considering revising the fines.