If the employee is unable to provide a certificate under Article 45 from the previous employer, what amount will they receive?

Only the annual taxable income from the current employer will be used.

However, if the employee knows they will receive the certificate later, it is better to submit an annual tax return directly. This way, they can use the total annual taxable income from all employers. Otherwise, they risk not being able to fully utilize the tax relief with their current employer, and the other parent may need to submit an annual tax return.