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

This looks like a mess. I would not pay him up front for the work unless the contract so states. Check out his BBB rating at bbb.com . If he has a good record, it might be worth the risk of paying up front. Otherwise, you may want to dole out cash as he cuts the rest of the tree. (Find something between your position and his). This is more of a transactional issue at this point than it is a legal issue. I would tell him you will make two installments (silly as it sounds). One more when he gets about half the remaining work done and another when he gets the job done.

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.