Answered by Shaykh Abdurrahman ibn Yusuf Mangera
A while ago I lent money to a brother, but after some time passed I had not asked him for it, I decided to waive the debt. However, he has recently given me the money that was owed.
Answer:
Assalamu alaykum
It is lawful for you now to keep this money and use it now since this was not a verbal vow. You will, insha Allah, receive the reward for having intending it as charity (sadaqa). If you wish, however, to donate it in the path of Allah for added reward you may do so.
And Allah knows best.
Wassalam