Will the reward in the Hereafter be diminished if a medical device is operated for a symbolic fee after the initial intention was for it to be for the sake of Allah, even though the money received is not personally benefited from?

It appears impermissible to impose fees on a donated device after the donation, unless the donor stipulated this from the outset. This is because attaching a condition after the contract has become binding is invalid, and because retracting a charity is unanimously impermissible. However, if the purpose of the fees is for the maintenance and repair of the device, then there is no objection to this, as it serves the common good and preserves the charitable work, and this is not considered a retraction of the charity.