What is the Sharia ruling regarding my money and the profits I earned from dealing with a person who later turned out to be a swindler? Am I obligated to compensate the charity with the money I dealt with him for the purpose of making a profit for it? Is it permissible to sell his mobile phone to pay off the charity's money, given the doubt about whether the phone is stolen?

You must guarantee the association's money and return its equivalent to its fund, because you are entrusted with this money and do not have the right to trade with it or invest it. An agent is held liable for transgression and negligence. While your intention to achieve profits for the association may benefit you in alleviating some or all of the sin, it does not absolve you of your responsibility to guarantee its funds. As for the second and third questions, these fall under matters of judiciary, not fatwa (definition), due to the need for investigation and reconciliation among creditors regarding the swindler's situation and his assets.