I want to know is it allowed employer can sign a...

The employer is allowed to issue an employment contract for the worker, and the employment contract must be authenticated by the Ministry of Human Resources and Emiratisation.
The worker and the employer must commit to implementing all provisions of the employment contract.
The worker is also prohibited from working for the employer without an employment contract, and the worker is also prohibited from working for a person other than the sponsor.
Here is the text of the new UAE labor law relevant to your inquiry:
Article 8
1. The employer must conclude an employment contract with the worker, according to the agreed-upon work pattern, provided that the contract is in two copies, the employer keeps one copy, and the other is delivered to the worker, and in accordance with the forms specified by the executive regulations of this decree law.
2. The worker or his representative may prove the employment contract, the amount of wages, and any of the rights he is entitled to under the provisions of this decree law, its executive regulations, and the decisions issued in implementation thereof, by all methods of proof.
3. The employment contract is concluded for a specific period, subject to renewal, as agreed upon by both parties.
4. In the event of extending or renewing the contract, the new term or periods are considered an extension of the original term and are added to it in calculating the worker’s continuous service period.
5. If the two parties continue to implement the contract after the expiry of its original term or the end of the agreed-upon work without explicit agreement, the original contract will be considered implicitly extended under the same conditions stated therein.
6. Taking into account Clause (3) of this Article, the executive regulations of this Decree Law specify the models of employment contracts according to the approved skill levels, and the provisions for changing the contract from one type of work to another type, and registering them in the Ministry.

The employment contract is often for a fixed period, and if you mean the probationary period in the UAE Labor Law, it is as follows:
The employer may appoint the worker under a probation period for a period not exceeding (6) six months from the date of starting work.
The employer may terminate the worker’s service during this period after notifying the worker in writing before (14) fourteen days
A minimum of one day from the date specified for termination of service.
A probationary worker may not be appointed more than once with one employer, and if the worker passes a period of
If the experiment is successful and continues to work, the contract becomes valid according to the terms agreed upon in it, and these must be taken into account
The period within the service period.
If the worker wishes to move, during the trial period, to work for another employer in the country, he must: •
Notifying the original employer of this in writing no less than one month from the date of his desire to terminate the contract, and the employer is obligated to
The new job shall compensate the original employer for the costs of recruiting or contracting with the worker, unless this is done
Agree otherwise.
If the foreign worker wishes to terminate the employment contract, during the probation period, to leave the country, he must notify his employer.
Doing so in writing, at least fourteen (14) days before the date specified for terminating the contract, and if he so desires
In returning to the country and obtaining a new work permit within (3) three months from the date of departure, the owner is obligated
The new work shall be subject to the compensation stipulated in Clause (3) of this Article, unless there is an agreement between