Is refusing to help complete the files of a charitable organization a sin, given a history of mistreatment, intermittent communication for work without a contract, and a secret request for help without the administration's knowledge?

There is no sin upon you for not assisting the employees of the charitable organization or for not teaching them, as that is not obligatory upon you. However, it is desirable for you to help them if you are able to do so without harm. Sheikh Ibn Uthaymeen mentioned that there is no sin in withholding what is not obligatory to give, but by doing so, one misses out on good.