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15 rubles and a cap.
He got 25 rubles from the neighbour and gave those back again, so that part can be ignored (the 25 fake rubles can be seen as non-money). It would be the same if he would just have had the change himself.
Apart from that, he gave 15 rubles and the cap to the "buyer", those are the things he actually lost.
Now, 25 rubles would be the wrong answer, since "10 rubles" and "a cap" are not the same thing - especially since only an idiot storekeeper would have paid the same price as he's selling the cap for.
So, the answer of what he actually lost in rubles is:
15 rubles + (10 - (markup on the cap))
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