Wholesaling tax certificates

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My friend acquired a tax certificate property for $5k. I'm about to wholesale a it for $15k. In this case, the owner has 6 months to redeem. From my understanding, the owner can buy the property back within the specified time frame(6 months), but has to also pay interest. So my question is, if a tax certificate was purchased for $5k and then whoesaled for $15k, does the owner have to buy it back and pay interest on the the $5k or $15k? 

My worry is that an owner may only accumulate a couple thousands of dollars in interest on the $5k it was purchased for and be able to buy it back for $7k. That would mean that the end buyer will take an $8k loss.