What might have been said
We're not going to try to appeal to your better natures, because you don't have them. Here is what's going to happen. You will treat the Viscount Potter as a normal child of his age. You will feed him as well as any other member of your family. You will shop for him as any other member of your family. If you cannot bring yourselves to buy him appropriate gifts for traditional gift-giving occasions, you will buy him gift cards to book or toy stores. You may assign the Viscount Potter such chores as are appropriate for a child his age. You may send him to his room and feed him iron rations for dinner if he misbehaves, but you will never raise a hand to or cause to starve the Viscount Potter ever again.
Because on the one hand, the stipend for fostering the heir of the house of Black is extremely generous and entails certain perquisites as well. And on the other hand, angering the house of Black has unfortunate side effects.
Dumbledore? The house of Black does not answer to Mr. Dumbledore and the political coalition led by the house of Black is one that Mr. Dumbledore needs to keep happy if he wishes to retain his exalted positions.
Now, let us discuss finances.
Yes, I believe we have an agreement. If you call your financial institution, you will find the first month's funds deposited. We shall upgrade the wards accordingly and be on our way. A pleasure doing business with you.