Nearly 10 months after his $10 million contribution to the Nicaraguan contras wound up in the wrong Geneva bank account, the oil-rich Sultan of Brunei wants his money back, with $300,000 in interest.

Judge Vladimir Stemberger, who is investigating how the $10 million found its way into the account of a rich Geneva businessman, said Thursday that the sultan had made a formal request for the money.

To complicate matters, the businessman who ''found'' the $10 million in his account at the Credit Suisse bank, and the bank itself, are entitled under Swiss law to be asked whether they agree to its return, Stemberger said. The interest is a separate matter.

''The sultan has asked for the return of the capital and the interest,'' Stemberger said. ''The capital is one thing but about the interest, that may be the subject of some litigation.''

The demand by the sultan adds a new twist to the bizarre aspects of the Iran-contra affair. He transferred the money to Credit Suisse last August at the request of Reagan administration officials, who appear to have forgotten about it promptly.

Because two numbers were transposed in the transfer form, the money went to the wrong account. The Swiss businessman who received it claimed he was expecting a large transfer from the sale of a ship, so he was not too surprised when it popped in. While waiting to make sure the money was his, Stemberger has said, the businessman put it in another account for

safekeeping.

Until May only the businessman, whose identity remains a secret, knew were the money was. No one else seems to have had a clue--not the sultan, the bank, the Reagan administration or the contras.

In a separate development Judge Bernard Bertossa of Geneva on Wednesday blocked the release of Swiss bank documents sought in the U.S. investigation of the Iran-contra arms and money transfers, legal sources said.

Bertossa ordered Credit Suisse and other organizations cooperating in the investigation to keep the papers until the Swiss Supreme Court rules on three appeals designed to block the move.

Retired Air Force Maj. Gen. Richard Secord and Iranian-born businessmen Albert Hakim and Manucher Ghorbanifar have gone to court to block release of the documents.

But Swiss officials nevertheless were willing to hand some of the information to U.S. authorities. Wednesday`s action will further delay the U.S. probe.