Merrill Lynch & Co. is involved in a dispute with an Indonesian private-banking client who is suing the Wall Street bank in Jakarta for $90 million in damages for allegedly selling shares without his knowledge.

Merrill, in turn, is suing the client, Prem Harjani, the owner of a Jakarta-based investment bank, in a Singapore court in an effort to recoup $12 million that it says Mr. Harjani failed to repay. This week, the bank won a court order freezing Mr. Harjani's assets in Singapore.