In July, trading house Itochu Corp, which already owned 50.1% of the FamilyMart chain, announced a 581 billion yen ($5.50 billion), or 2,300 yen per share, tender offer for the rest of the chain. Itochu expects to delist the retailer if the offer, which runs through Aug. 24, succeeds.

Itochu has said 2,300 yen was the best it could offer.

Critics such as Oasis have said FamilyMart should have demanded a higher value, and that by not doing so it prioritised the interests of major stakeholder Itochu at the expense of others.

Oasis said FamilyMart turned down an earlier call for a dividend, but that it was standing by its demand.

"By not paying out the dividend, you are not treating all shareholders equally," the Hong Kong-based fund said in a letter to FamilyMart, which it made public.

"Instead, you are facilitating a bid that is advantageous for one shareholder at a price that's unfair, and rejecting a special dividend for all shareholders in order to facilitate that bid," it said.

A FamilyMart spokesman said he could not immediately comment.

($1 = 105.6200 yen)

(Reporting by Ritsuko Ando; Editing by Muralikumar Anantharaman)