Is the excess amount of money I received due to a contract error considered unlawful gain?

As long as the contract stipulates the aforementioned substitute, there is no harm in collecting it in full, for the contract is the law between the contracting parties. The questioner has the right to demand the difference between the value stipulated in the contract and what she actually received. What her colleagues informed her of is not binding on the questioner, as they are not parties to the contract, and the existing contract remains in effect unless it is annulled or amended with the consent of both parties.