What is the ruling on taking a sum of money from a company by mistake after it caused a reduction in the previously agreed-upon prices of works, and is it obligatory to inform the company of this error?

If additional works are undertaken without an agreement on their price, the ijarah (contract of hire/lease) is invalid, because specifying the price is a condition for the contract's validity. In this case, you are entitled to the ujrat al-mithl (customary or fair wage), which is determined by experts. It is permissible to take what corresponds to the ujrat al-mithl from the amount paid and return the excess to the company. If the amount paid is less than the ujrat al-mithl, you may demand the remainder. The resolution of these matters falls within the purview of the Sharia judge.