Thank you for your question. I am a licensed New Jersey professional. Please permit me to assist you with your concerns.
There are two separate issues here, and please permit me to respond to both.
There are actually two potential breaches of contract here, one that he committed against you, and one you committed against him. In terms of his potential breach, if he requested more funds for the same exact work, it may be a breach of the original terms. Likewise, requesting that you purchase the top at your own expense without benefit could be a breach of terms. He could request or ask that you purchase it on your own, but would not be able to charge you for value out of the original contract.
On the other hand you could not charge him for your time, gas, travel, or pain and suffering--this is a contract, and unless he expressly contracted with you for chose charges, you could not pursue them against him. Furthermore, New Jersey law does not grant 'pain and suffering' for a contractual breach, but for personal injury claims, something that did not happen here. If you failed to pay him based on this withholding, the courts would not deem it be reasonable and would request and require that you pay him what he earned, minus value for the top itself.