Your information about the Cadillac is a little off, Henry Ford, and Ford Motor Company did not sell the car company to General Motors, he was, however, involved in developing the original design, but never built the car, it was built by Henry Leland.
In 1902, before he succeeded in bring his Model T to market, Henry Ford was part of an automobile venture that failed. The major investors in this venture brought in an expert to sell the assets of this failed company, his name was Henry Leland.
Henry Ford continued in the car business, successfully launching his own company, Ford Motor Company in 1903.
"After two unsuccessful attempts to establish a company to manufacture automobiles, the Ford Motor Company was incorporated in 1903 with Henry Ford as vice-president and chief engineer."
It was Henry Leland, with the assistance of Henry Ford's major investors from his first failed attempt at car building, who developed the first Cadillac and brought it to market in 1903.
"Henry Leland introduced the $850 Cadillac at the 1903 New York auto show. Within a week, he'd taken 2286 orders. Cadillac built nearly 2500 cars that first year--a huge number at the time."
General Motors bought Cadillac from the venture capitalists, retaining Henry Leland as president of the company in 1909.
By the time General Motors bought Cadillac, Henry Ford was no longer involved with the car company that developed it. By 1909, Henry Ford was enjoying his own success with the Model T.
"Henry Ford realized his dream of producing an automobile that was reasonably priced, reliable, and efficient with the introduction of the Model T in 1908. This vehicle initiated a new era in personal transportation. It was easy to operate, maintain, and handle on rough roads, immediately becoming a huge success."