Is it permissible for me to take the price of selling the scraps from cutting the curtains that I tailor, knowing that the customer is the one who buys the fabric?

If the customer leaves fabric remnants for you, knowing that they are doing so, then it is permissible to sell them and take their price. However, if the customer is not aware of this, then it is not permissible to sell them except with their permission, unless it is customary for the customer to leave any remaining fabric for the tailoring shop. In this case, custom takes the place of permission, as the Prophet, peace and blessings be upon him, said: "The wealth of a Muslim person is not lawful unless given with a willing soul."