Please advice me!

My sister needed her phone topped up so I went to top it up for her. I gave the guy 20 pounds, somehow he ended up putting 40 pounds credit on my sisters phone. My sister asked me if I put 40 pounds? I didnt, i'm very sure that I gave him 20 pounds. We came to know of this only after we arrived home hours later. My sister says its not our fault, we gave him 20 he put 40 its not our mistake.

couple of days later, i'm now worried what if Allah is trying to see what I will do. I was the one who paid the money, so will it be my sin? I told my sister that I will pay the guy a visit and give his 20 pounds back, but sis says how? The money was not cash he put as credit on her phone. On the receipt it also says that I paid 20 pounds. I'm confused. I dont want to be a thief in the eyes of Allah. Please tell me what I can do. Jzk.