All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We will answer your question in the following points:
1- In principle, cash currency should be valued by gold and silver as stated by the contemporary scholars, and these days it is better to measure it by gold as it is more constant in its price; the Nisaab (the minimum amount liable for Zakah) of gold is 85 grams. Therefore, if the money equals 85 grams of gold then Zakah must be paid on it.
2- If the money reaches the Nisaab and a whole lunar year elapses on it, then Zakah must be paid on it. Therefore, if the first amount of money – which is 2500 dollars – reached the Nisaab and a whole lunar year elapsed on it before giving it to your brother, then you must pay Zakah on it.
3- The money that you have with your brother is considered as a loan (a debt on his liability) and if the debt is with a rich person who acknowledges it and is able to pay it back, then Zakah must be paid on this money if it reaches the Nisaab by itself or with other money added to it provided that a whole lunar year elapses on it.
4- As regards the money that you got afterwards: then it could be either: A) a profit of other money, then its lunar year is the same as the main money, or B) new money which you possessed independently, then if it reaches Nisaab by itself, you may make a separate lunar year for it, or rather you may pay the Zakah of all the money you have when the lunar year elapses on the first money as a way of hastening to pay the Zakah on the part of the money which you got later, and this is permissible according to the preponderant opinion of the scholars. Of course, this is easier for the owner of the money and more beneficent for the poor. For more benefit, please refer to Fatwa 92234.
Finally, it should be noted that once a sale agreement is concluded and the agreement fulfilled all the pillars and conditions of valid sale, then the contract becomes binding and neither party of the contract is permitted to cancel it. However, if one party to the contract asks the other party to invalidate the sale contract between them, then it is desirable for the other party to invalidate the sale with him, as the Prophet said: "Whoever frees a Muslim from a commercial obligation, Allaah will forgive his sins on the Day of Judgment.” [Abu Daawood and Ibn Maajah]
Allaah Knows best.