Pidyon Money Later Found to Be a Mistake

I am a Kohen, and I performed a Pidyon Haben for someone, but it turned out that his wife is a Leviah, which means they were exempt from the Pidyon. He immediately forgave the money that I had received from him.On the other hand, I am concerned that until he forgave it, the coins were not really mine, and in the meantime I used them to perform Pidyonim for other children.What does the Rav say? Thank you.P.S. Please also explain a bit of the reasoning in the answer.

Shalom UvRacha,

There is no problem, since the father forgave the money, that forgiveness works retroactively, so it turns out the money was yours all along. 

According to the psak of our Rav,  HaGaon HaRav Amrom Fried shlit”a.