Eastvaal motor citys mistake affects service plan

I bought a car from eastvaal motor city in witbank last year on 71000km.I took out a extended service plan wich i was told on the day of signing contract it is efective from 79000km also stated like that in contract.I took the car for service on 79500km at kholers whereby the service plan refused to pay because the 75000km service was not done or stamped in service book.I then contacted the people at eastvaal motors and told them about whats going on the told me that it was a fault from the previous owner of the car .I told them how could it be because i bought the car from them on 71000km where by they denied it and said there system shows that i bought it on 79000km.After a big argument and proof of when they booked it in at their workshop to fit new wiperblades before delivering the car to me on the day of purchase the job ard showed that it had 71000km on.I even went to pc glass for the insurence inspection print out to also proof the km which also confirmed the 71000km.Now they admit that there was a mistake made by them on the contract and that they would contact the service plan company to adjust the km on effective km to 75000km from 79000km.BUT i have to pay now to get the 75000km service done for the service plan to stay active because they say i had to take the car for the 75000km and its my own fault now.