Abby enters into a contract with Bill for Bill to renovate Abby's bathroom and kitchen for $50,000. However, once Bill started the work, he realizes that it will cost an additional $10,000 to complete the work that he promised. To show good faith, he tells Abby that he will split the added cost with her and asks her for an additional $5,000. Abby refuses to pay the additional $5,000 and Bill threatens to stop work on the renovation. But realizing that another contractor will probably charge much more, she agrees to the additional $5,000. After the renovations are done, Abby again refuses to pay the additional $5,000. Bill sues Abby.

Who wins? Please explain.