To what extent is the Hajj of one whose employer bore half of his expenses, while the worker paid the other half in installments, considered valid?

Your Hajj performed in this manner is valid and there is no harm in it, if you are able to pay the installment from your salary. Otherwise, it is better for you to postpone Hajj until Allah makes it easy for you. The half of the costs borne by the employer is considered a gift, and there is no harm in accepting it and performing Hajj with it.