Is it permissible to purchase a device to unlock a mobile SIM card specified by the company for use, knowing that exceeding the set limit will result in a deduction from the salary?

It is not permissible to use the phone beyond what its owners have authorized. Otherwise, you would be transgressing, betraying a trust, and unlawfully consuming people's wealth, as Allah Almighty says: ﴿And do not consume your properties among yourselves through falsehood﴾ [Al-Baqarah: 188], and as the Prophet, peace and blessings be upon him, said: "Every Muslim's blood, wealth, and honor are sacred to another Muslim." Using the SIM card outside of what was agreed upon is forbidden.