Please sort out your store policies or return my money.. I'm unemployed

Please assist,

I bought a Vodafone smart kicka at Ackermans Aliwal North for my daughter last month. She has not use the phone because we received a ********** Vodacom sim card with it which came owing before it was even used. The more I tried recharging the more I received notifications that there was an outstanding amount. Before I could replace the sim card, phone could not charge any more. I bought another sim card hoping it won't have any outstanding amounts owing and had inserted it without any problems, phone still could not charge. The phone has a year of warranty and I tried taking it back. There was no manager yesterday and I had to leave the phone in store and had returned to the store today. All of a sudden the phone has a bit of oil looking substance on the surface of the phone only visible when one opens the cover of the phone, I don't understand how that got there. Today I'm being shown there's oily looking marks under the cover and for that reason the device was rejected. I believe it is a technical fault as the battery is not charging but indicates when a charger is connected. But at the store they decided to make their own discretion to say they will not send the phone in for observation because there was oil. Who's responsible for the oil on the cover? Definitely not me, coz I would have seen the oil marks outside the device as well if it happened that it had landed on some oil at home, or when I opened the cover to exchange sim cards before taking the device to the store. I don't even own a bottle of oil at home. R400 is a lot of money for me to accept the store's response on this matter, I am unemployed and my daughter needs the phone. Please advise the steps to follow with the store for the device to be taken back for observation because the problem is with charging and not water damaged. I would also like to see proof that the device is water damaged and not technically impaired. Your urgent response will be much appreciated.

Regards Luyanda