FAQ for
The Last Picture Show (1971) More at IMDbPro »

It is never clearly explained. In the novel there is a reference to a time Sam accidentally threw a bucket of urine all over Joe Bob and leaving the money was a way of making amends, although that seems far fetched. Some have speculated that perhaps Sam, who had a history of drinking and having affairs with married women after the deaths of his sons, was in fact Joe Bob's biological father and left the money as a result, but this is purely speculation.