Candy has alot of money saved up. He was given $250 as compensation when he lost his hand on the ranch. He has another $50 saved from working and expects another $50 at the end of the month. So, he had $350.
Candy very much wants a dignified and comfortable way to live out his life once he can no longer work, which he expects to be soon. He offers to contribute his $350 toward the $600 needed to buy the land, and says he'll write a will so his interest in the land will pass to George and Lennie when he dies. With the $100 George and Lennie would get at the end of the month, they would be able to get the land. George and Lennie would move to the ranch and get it started, and George would get a job to earn the balance of the $600.
Candy's financial contribution makes the dream of buying the land seem really possible for the first time.