Pieter Malan has left Middlesex a month earlier than planned to re-join the South African domestic league.
The 34-year-old batter joined the club midway through last season and helped them secure promotion back to Division One of the County Championship.
He scored two-half centuries and 200 runs in their last five four-day matches, as well as 394 runs including a ton and four fifties in one-dayers.
But he has struggled to repeat that form in eight matches this season.
Malan has scored 221 runs with a highest knock of 66.
"Pieter is a wonderful human being, with an incredible work ethic," Middlesex director of cricket Alan Coleman told the club website.
"You'd struggle to find a player that works harder on his fitness and on continuing to develop his game, however he would be the first to admit that this season has been a struggle for him on the field.
"Pieter leaves Middlesex having made many friends. We wish him every success in the future, and have little doubt that it will only be a matter of time before he is back in the runs for Boland."
Malan will captain Boland in the CSA 4-Day Domestic Series.
Middlesex, who lie eighth in the Division One table, will look for a replacement to help them in their four remaining matches, starting against Essex at Chelmsford on Monday 4 September.