Recently, I got a 10 AUD banknote from someone and as a courtesy, bought him coffee(costing me more than 10 AUD)
In another instance, i got 50 Mexican Pessos from someone and again as a courtesy, I gave her 100 units of another currency which I had multiple copies of in my currency collection.
My question is, if some day I decide to use this money for my expenses, will it be halal for me? I am sorry if the question makes less sense but I was thinking that I bought money with money which should not be halal. Or?