Sam offered to sell his CD collection to Sandy for $100. Sandy replied, "I'm interested. I'll think it over and let you know Thursday whether I want to buy the CDs." On Wednesday, Sam agreed to sell the CDs to Jason, and Jason immediately gave Sam a letter that stated: "Sam, I will buy your CD collection for $100. As we agreed, I will pay you on Friday when I pick up the CDs. Yours truly, Jason." Upon Sam's receipt of this letter on Wednesday, what best describes Sam's contract agreements