Contract law

Your condominium is downtown, and the prostitutes linger just outside your private garden area. You decide to have a tall privacy screen built, which would be on your own unit’s area, not the common area of the condominium complex. You enter into a contract to have it built, and half-way through building the screen, your contractor tells you he needs more money to finish the project as it is taking longer than he expected. You agree to pay him more, with no other changes to the contract. When the screen is finished, you only pay him the original sum due. He says he will sue you for the full amount. You say the contract wasn’t in writing anyway. Explain each parties’ respective positions and authority for the position, and who you think is right and why.