Pos error counting

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We tried to make a sale yesterday throigh pos but unsuccessfully. We entered the products at checkout and on each product we a reduced price. So what we did is that instead of changing the price from admin we just placed the reduced price while we were making the sale on the ipad. But, the system was counting totally wrong and was giving totally wrong amount, the client should pay!!!! We have tried this 5 times and 5 times was counting totally wrong!!! Bear in mind that the client got irritated and she left. :( Can someone explain why is this happening and how can i solve this on the spot without any delays?