Is anything required for throwing the pebbles prematurely on behalf of someone else during Hajj, while intending to delay it?

If you performed Hajj on behalf of someone else and accepted the condition of the delegator to stay longer in Mecca, then you must fulfill this condition. If the Hajj was completed, it is valid, but you should apologize to the delegator and explain the reason for your hastiness.

As for the money: - If the money was for the delegation itself: You should spend from it for your Hajj and return the remainder, unless the delegator permitted you to keep it. - If the money was for a service/hire (which is permissible according to the Maliki and Shafi'i schools of thought): Then any surplus is yours, but the hirer has the right to deduct an amount equivalent to the day you did not spend in Mina.

Therefore, you should seek forgiveness from the delegator or hirer and offer to return part of the payment, especially if you did not inform them about the campaign's accelerated departure.