It follows a request by Cape Town-based Western Province, where 32-year-old Carstens will now head in time for the new Super Rugby season.
"We have a partnership with Western Province rugby, and any partnership must work both ways," Saracens rugby director Mark McCall said.
"The Stormers urgently need a top-class international loosehead prop for the forthcoming Super Rugby season. The move makes sense for the player, and so we have agreed to the release.
"Deon has made a magnificent impact at Saracens over the past 18 months, and his departure presents us with some difficult mid-season challenges, but we wish him, his wife and young daughter all the best in Cape Town.
"Since this is the right decision for Deon and his family, it must be the right decision for us."
Carstens, who joined Saracens last year, added: "I am very sad to be leaving Saracens, but I am massively looking forward to the opportunity in Cape Town.
"I am also very grateful to Saracens for their pragmatic approach to this situation."
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