Nguyen Khanh Hung said he had been on a business trip Tuesday and instructed a secretary to inform the Ho Chi Minh Stock Exchange (HoSE) before making the sale.
The person, a new recruit, delayed making the disclosure, he claimed.
The shares belonged to him and not the company, he added. The sale on Tuesday was worth VND16.7 billion (US$699,800).
HoSE has not announced the penalty yet but canceled the transaction on Wednesday.
LDG, established in 2010, develops housing in Ho Chi Minh City and Binh Duong and Dong Nai provinces.