Improper Alignment that cause damage to my tyres

I took my car BMW 320i M sport for alignment and balancing to Tiger Wheel and tyres in Witbank and they didn't adjust it properly on the rear wheels and that resulted into my tyres being worn on the inner section of the tyre width /- 50mm areas got damaged and now it showing the wires on the tyres. they have acknowledge their fault but they say one bolt was lose on left hand rear wheel suspension so they will pay for that tyre only. i must pay for the other tyre of which i'm disputing that and they must replace both of them cause the damage is on both rear tyres. they are refusing and say it a mandate from head office in Gauteng