Marseille finished last season in mid-table after a disappointing Ligue 1 campaign, and Labrune has revealed that the club must sell in order to buy.
"For the next three weeks, as it has been since the start of the transfer window, our objective is very clear - we must sell players," he told France Football.
"If Chelsea makes a fair offer for Aspi, we will let him leave. If there is a good offer for Stephane Mbia, we will let him leave too. We are working in good terms both with Cesar and Stephane."
Loic Remy could also leave the club this summer.