The 18-year old midfielder has been heavily linked with the club for a while now and according to German daily Bild, he is on the verge of signing a four-year deal.
Amini said he will soon sign with the club but will spend the next A-League season back on loan with the Mariners so he can develop further.
"(Dortmund coach Jurgen) Klopp told me that it was better for me if I play another year in Australia, so I can prepare myself quietly for the U-20 World Cup in Colombia and the 2012 Olympics in London," Amini said.
"In addition, I can also accumulate match practice in the next Asian Champions League.
"Coach Klopp took me aside and told me that I had convinced him at the trial session with my style of play.
"In general, he treated me as a full team member all the time. He's a really extremely nice guy - and an absolute professional."