MEXICO CITY, July 22 (Reuters) - Mexican billionaire Carlos Slim's financial holding company Sociedad Inmobiliaria Carso will make an offer to buy the outstanding shares of real estate unit Inmuebles Carso.

Sociedad Inmobiliaria Carso has offered to pay 15 pesos a share for the 19.55 percent of Inmuebles Carso's outstanding stock, according to a statement from Inmuebles Carso.

That represents a 12.4 percent premium to the stock's closing price on Friday. The shares, which trade infrequently and with little volume, did not trade at all on Monday.

Inmuebles Carso operates hotels and also owns the buildings that house Slim's Sanborns Cafe chain and other retail businesses.

(Reporting by Elinor Comlay. Editing by Andre Grenon.)