Is it permissible for an employee who works for a fixed salary in a real estate office, and is required to give the office a small percentage (10%-20%) of his external work, to perform this external work for his own account without giving the office a percentage, knowing that he does not agree to the specified percentage and completes his primary work in the office to the fullest extent?

The question includes two matters: receiving a percentage for work done outside the office, and the ruling on your private external work. There is disagreement regarding the permissibility of a percentage as wages. As for external work, if it is part of the agreed-upon contract, then it is obligatory upon you and can only be changed by a new contract. If it is not part of the contract, but was done during work hours or using the employer's tools, then it is not permissible except with the permission of the employers. This is because a private employee is bound by the terms of the contract, as Allah Almighty says: "O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS."