Spencer promised to pay Georgina $4,000 for building a deck adjacent to his house. The terms of the contract included the dimensions, the type and quality of lumber, and a completion date of May 15. On May 1, Georgina said to Spencer, "Since I know that you need it finished earlier than May15, if you pay me an extra $500, I will be out of here by May 10." Spencer agreed, as he was worried that it may not be completed in time for the family reunion he was planning to have. If the deck was completed by May 10, how much is Georgina legally entitled to receive from Spencer? Explain this fully in terms of contract law principles.