The price of water

I could hear the asthmatic water truck make its way up the hill and what a stroke of luck that was. I was down to the last few liters of bottled water in the big blue 18 liter water bottles I keep for drinking.
I asked the guy for three and feeling grateful and generous and happy not to have to haul water bottles up the hill, I gave the guy a 200 peso bill and said
" just give me change for ninety"
he grinned at me and said   "ninety ? " 
I agreed and he handed me a wad of bills which I stuck in my pocket and with a big face cracking smile he  got the truck started and took off as I struggled the bottles into the house.
The bottles taken care of I put the money back in my wallet and realized why he had the big grin on his face.
He had--maybe deliberately ?-- misunderstood and just given me ninety pesos in change.
Some expensive bottles of water.
Next time I will count the change before he leaves.
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