Maths Question

got 1 thing cost is Rm19, then i sell it for Rm21 .. then got 1 guy give me RM100 to buy the thing, but i no change, so i exchange the Rm100 from 1 person , then give back the balance money to the buyer.... Then after that guy left , the person that i exchange the money say to me that the Rm100 is fake money , and he request to give back him his RM100 , So question is , how much money do u lost?

if can give me the answer by comment it or chatbox there, thanks.