The Hungary eventually left Salzburg for RB Leipzig before being sold to Liverpool.
But Boban told the Milan Hello YouTube channel: "We closed Szoboszlai.
"Everything was agreed: 20 million (euros) for the clause with Salzburg. They (Milan's board) rejected that and I was like: 'What the hell is this?'
"Then I tried to meet and they didn't want to see us for two months, so I had to do what I did. For some it was sudden (his departure), but it wasn't really sudden. But we couldn't say things publicly every day or push to meet with them for a clarification that never came.
"Szoboszlai shut himself away in Innsbruck; Paolo (Maldini) didn't go for fear of being recognised. He's an icon. Wherever he goes, he's recognised.
"So Ricky (Massara) and I went with Szoboszlai's father. We had closed the deal; the boy wanted to come immediately, but they (Milan) turned it down. I had to tell him: 'Look, let's see what happens in the summer'.
"He was very disappointed; he wanted to come to Milan immediately."