August Alsina Refused to Shake Tory Lanez's Hand Before Alleged Assault

August Alsina isn't denying he left Tory Lanez hanging during an attempted handshake -- but this new video shows the diss happening, and Tory's immediate and angry reaction.

You can see August strolling through a backstage hallway in the Chicago venue ... when Tory extends his hand out for a shake, AA hit him with a fierce side step to avoid all contact.

A perplexed Tory was left standing in the hallway with his hand out ... until an onlooking fan dapped him up. After shaking off the embarrassment, Tory and his security team regrouped before squaring up with August again where the alleged assault took place.

As he's prancing through the hall, you can hear a woman pleading for Tory not to "do it" because of his pending case against Megan Thee Stallion.

According to August, Tory "did it" anyway ... as the video shows his security rushing him from the area and later on, giving high fives as the cameraman talks about him knocking out AA.

As we reported, August later hit social media to post pics of his bloody mouth and scraped skin, accusing Tory and his team of the brutal beatdown while also daring them to release footage he says they have of the whole thing.