Is failing to return money to a disbelieving woman who caused discord considered a sin, especially given the absence of a prior agreement to return it?

There is no harm in taking a commission fee (brokerage) or a stipulated fee (ju'ala) if it is for a permissible matter, and a condition for its validity is knowing the amount of the stipulated fee. It is not obligatory for the one who took the fee or commission to return it, as long as they have performed the agreed-upon work. So, if those who mediated between your colleague and the domestic worker have performed their work according to the agreement, they deserve the amount and are not obliged to return it, even if the woman dispensed with the domestic worker. You are not required to demand that from them, and if your colleague has a right, she may refer to them.