old plug in tyre caused slow puncture which cost me a new tyre

We picked up a car in George to Grootbrak - next morning the tyre was flat - we have put the spare on - drove back to find out what do do! NO problem they switch vehicles immediately - My question will we pay - no mam it is an old tyre plug that caused a slow puncture! after our return struggles 3 days to get deposit back but was informed that the R1500 that they have deducted is for the tyre - another R203 was for petrol- I have locked 2 cases and both was closed as we need to pay! I can buy a new tyre for that vehicle with R1500 and would have fixed it for R45 if they told us that! the branch manager is trying to help but seems the head office are the ones calling the shots and what a expensive experience! I will never use First Car Rental again!