What is the Sharīʿah's ruling on not disbursing an employee's dues (bonus) on the grounds that he resigned before they were disbursed?

Your entitlement to the bonus depends on the employment contract and the agreed-upon terms. If it was a gift from the company, then the company has the right to retract it. However, if it was due as compensation for extra work or a commission, then the company is not allowed to withhold it, unless your resignation constituted a breach of contract. You can file a complaint with the responsible authorities to claim your right.