We had an oral agreement with potential tenants that they could make some improvements to the house in exchange for a few month's rent. The tenants proceeded to spend over $10,000 on improvements without notifying us. Before moving in, they decided to rent elsewhere, and are now asking us to reimburse them for everything they have spent thus far. The work is not complete, and it will cost us at least two thousand dollars to make the house liveable for new tenants. What is our obligation?