Am I sinful for returning the maid to the recruitment office after four months of trying to help her adapt, knowing that my daughters were uncomfortable with her presence due to her lack of cleanliness and absentmindedness, and I paid her full salary and secured her in a good office?

If the contract between you and the maid is on a monthly basis, then there is no blame upon you for what you did. However, if the contract is for a specific period, it is not permissible to terminate it before its expiry without the maid's consent, unless she has a significant defect that diminishes her benefit or causes a disparity in the wage.