The Russians will give Sundowns a 'take it or leave it' offer for the centre-back, with the two clubs having haggled over a price for his services over the past week.
Rostov put in a bid last week, which included a percentage of any future sale. Sundowns have countered with a higher figure that is likely to be rejected by the Russians, who say they will make one last attempt to lure Sundowns into selling with a revised offer on Monday.
Insiders say the issue will be sorted out once and for all today, with Rostov ready to accept either a 'yes' or 'no' from The Brazilians and move on.
The 20-year-old spent a week on trial at Rostov last month and the Russians were impressed with what they saw.
This follows trials with Barcelona and Arsenal last year, while Spanish La Liga side Atletico Madrid made a bid for him ahead of last season without even seeing the player in training. Sundowns rejected that offer.
The Chloorkop side have already signed replacements for Xulu, bringing in central defenders Wayne Arendse from Santos and Dutchman Arle Schut from FC Utrecht.
Together with Clayton Daniels and Method Mwanjale, The Brazilians now have four centre-backs at the team, which should smooth the exit for Xulu.
Rostov is a club that knows South African players well, having previously had Matthew Booth, Tony Coyle, Rowan Hendricks, Bennett Mnguni and Japhet Zwane on their books.