Will an employee be sinful if he refrains from performing additional tasks assigned to him after the employer fails to pay an agreed-upon bonus, knowing that his refusal will delay financial dues and claims for other people?

The contract of Ijarah (leasing/hiring) is among the obligatory contracts. However, if the employer breaches the agreement, then it is permissible to cease working, and there is no sin upon the one who ceases. Rather, the sin falls upon the employer.