Asked to pay extra for first time Sim card buy

I brought new O2 simcard in Stuttgart, with the plan 9.99 Euro but I was asked to pay 20 Euro. Sales person told me that the rest 10 euro you will get subsequently in next month.but I cannot see any such balance in my account apart from the current plan. And Moreover my friends also brought O2 simcard with same plan from Milaneo mall (Stuttgart) they paid only 7.99 Euro for 9.99 euro plan. I think I have been overcharged.please clarify 

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