The man behind the suit is Todd Mustin, who says back in 2017 ... he put his 5-bedroom, 5,000 square-foot Sherman Oaks, Calif. pad up for sale -- when things went awry.

Mustin claims he sold the house to a woman named Megan Zucaro ... but when she only coughed up $1 million of the roughly $1.9 mil she agreed to pay for the place, Williams stepped in.

Mustin says the 66-year-old -- who won an NBA title with the Sonics in the 1978-79 season -- agreed to take over the place and pay off the remaining $900K.

The problem? Mustin says the former point guard NEVER paid a dime ... and claims the guy and his family trashed the crib in the meantime.

Mustin claims Williams and his family left the place an absolute dump ... telling TMZ Sports they left feces in the toilets and trash all over.

To be clear, Williams had every right to trash the place -- he did buy it after all -- but Mustin claims the disaster he left behind on the property proves the dude is a complete jerk.

Now, Mustin claims Williams owes him $900K -- plus interest -- for never paying for the house after he agreed to take it off Zucaro's hands.