1. Both parties sign and seal the contract, which is usually understood as the establishment of this contract.

According to your description, the other party only has the signature of the employee.

If the employee is authorized or ratified by the company afterwards, then the contract is established, which does not belong to the situation of unauthorized or ultra vires agency signing.

In view of your present situation, I suggest:

(1) and the company negotiated supplementary terms and sealed.

(2) If the other party disagrees, it will notify in writing to terminate the contract, because the signatory of the other party has not obtained the written authorization of the company.

In fact, the decoration company belongs to the construction unit.

In practice, construction needs corresponding qualifications, and some unqualified units have not shirked their responsibilities and will choose not to stamp. You'd better know the specific situation to avoid future losses.