RJR Nabisco Inc. management sweetened its offer to buy the food and tobacco giant, topping a rival bid by a New York takeover specialist in a battle for one of the nation's best-known consumer products companies. The management team led by chief executive F. Ross Johnson announced an offer Thursday of $84 per share in cash and $8 per share in securities, worth about $20.7 billion. The total $92 per share tops a $90-per-share, or $20.25 billion, tender offer made by Kohlberg Kravis Roberts & Co. and surpasses the management group's initial overture for a $75 a share buyout.