Is the thousand that was taken considered unlawful (haram), after purchasing an undivided plot of land for 8,000 and registering it for 9,000, then having the major partner file a lawsuit to forcibly purchase the land, and winning the case by depositing 9,000 in the court?

What the seller's partner did is pre-emption (shafa'a), which is the forced acquisition of the pre-empted property from the buyer for the price and expenses he incurred. Therefore, you do not have the right to increase the price beyond what you paid to the seller and what you spent on the land. If you did not spend anything more than the eight thousand, then the extra thousand is not yours. But if you spent anything extra, even one riyal, then you have the right to take it, and pay the remainder to the one to whom the land has transferred.