I hired a tree trimmer to trim a large tree. He did half of the work and wants the full amount.

It depends what your contract states, and it seems that you should have an attorney review it before you decide what to do. An attorney's advice can be very valuable in these instances. If the first contractor agreed to do the whole job for the set price, and is now refusing to preform at that price without a valid basis to do so, you are in the right. You would then need to send notice to that contractor in writting that they must preform or you will have someone else do so and then plan to charge them back the amount it costs to complete the job. Expect them to dispute your claim. It sounds like they think they have done the agreed upon work, or feel they are entitled to payment up front. If it does read this way, things may turn out different. Be careful before you hire someone else.

Be sure to document your actions, and to document and have proof of the necessity to have the work done on your specified timeframe. You will need this proof to back up your claim.