Mannat means prayer or a vow made to the lord, after fulfilment of which a person makes offerings to the Almighty God.
A devotee of the lord however asks nothing in return, just out of his natural devotion and gratitude towards the saint, he wants to offer something to him. They offer the money in the consciousness that someone who is really in need and is entitled to it will get helped by this
A person making a vow that he will offer some money to the dargah should fulfil his promise. Even if you don’t have any specific project in his mind he should be serving the Dargah requirements like shrine refurnishing activities, repairs in the dargah, new installations, flowers, chaddars, etc.