Have I or the accountant committed a sin; because I worked for a person and agreed with him on a certain amount, but he reduced my wage, so the accountant gave me the agreed-upon wage without the employer's knowledge and without recording it in the books?

If a worker agrees with an employer on a specific wage, and the worker completes the work in full, then he is entitled to the full agreed-upon wage, and the employer is not permitted to reduce it. In the Hadith: "There are three whose opponent I will be on the Day of Resurrection... and a man who hires a worker and benefits fully from him but does not give him his wages." If the employer denies the worker's right and the worker is unable to obtain it through known means, then he may take his right without the employer's knowledge. This falls under the "issue of gaining possession of one's right." There is no sin upon the worker or anyone who assists him in taking his right.