Descendants of Levi Strauss & Co.'s founder offered yesterday to buy back the giant jeans company from stockholders for $50 a share.

The deal to take Levi private would involve more than $2 billion, including $1.45 billion in financing. A group of family members, including some of Levi's top executives, has received a commitment for that amount from Wells Fargo Bank and Salomon Brothers.

Robert D. Haas, great-great-grandnephew of Levi Strauss and the company's president and chief executive officer, made the offer to a special meeting of the board of directors. He represents family members, who already own about 36 percent of the company's stock. Trusts controlling another 4 percent are expected to support the bid.

Levi has suffered sluggish sales and earnings since 1980. Sales topped $2.8 billion in 1980 and 1981, but were down to $2.51 billion for 1984, while earnings dropped to $41.4 million last year, from $194.5 million in 1983. Plant closings and layoffs were blamed for much of the earnings decline.

Haas said the group was making the proposal through a newly formed company called HHF Corp., "to better enable management to focus attention on the long-term interests of the company."

Levi's stock had been hovering in the mid-30s before buyout rumors surfaced last week. By yesterday's close, the stock had risen to 47 1/8, up 1 1/2 points on the day with 281,000 shares trading hands.