Is it permissible for the donor to reclaim the amount he donated for building the mosque, given that the committee has not started the project yet? And is he who used this amount in his trade committing a sin, and how can he absolve himself?

Thank you for your loan and donation to the mosque, and we ask Allah to reward you. What you have provided is a loan and a donation to the mosque. A loan is what is repaid, and a donation is what is not repaid. You have the right to reclaim the agreed-upon amount due to the committee's failure to adhere to the agreement, because the amount is a loan. As for the donation, it is not binding on you unless you had intended a specific amount, in which case you may reclaim it and give it to another committee or a trustworthy person responsible for the mosque. If the person who took the money invested it for their own personal gain, their action is forbidden (haram) and they must repent. If profits resulted from their investment, the Shafi'is and Malikis held that profit follows effort, and according to this view, only repentance is required of them. The Hanafis and Hanbalis held that profit follows the capital, and according to this view, they must return the money along with the profit.