Unless you have a clear contractual entitlement to a specified level of bonus or commission, your employer may be tempted to withhold payment.
For example, your employer may claim that any bonus is discretionary rather than a contractual entitlement. But if it is normal practice for your employer to pay bonuses, you may be able to argue that they have in fact become an entitlement — regardless of what the written contract says.
Another common situation is where the contract states that bonuses are not payable to employees who are on notice, or have stopped working for the business. Sometimes contracts distinguish between ‘good leavers’ — who have to leave for health reasons, for example — and ‘bad leavers’ who have been dismissed. And it is not unheard of for employers to time a dismissal in order to avoid paying a bonus.
Again, you may be able to argue against this. For example, you might not have been dismissed for a fair reason, or following a fair procedure.
If you have been dismissed without being paid the bonus or commission you think you are entitled to, you should take legal advice. It may well be possible to negotiate a settlement agreement with the employer, where they agree to pay you some or all of the amount you expect.