It is unlikely the garage who sold it actually meant to mislead - the consequences of a snapped cambelt shortly after sale would not be worth their while. Possibly the previous owner has told them this, mistaking the aux belt for example. Most people are 'tards as far as cars go and wouldn't know one from the other.
In my opinion, the garage are rather kindly offering to cover their good faith error. If it had been 3-6 months down the line, the full price should have been covered without question. This is more than 2 years ago, you are fairly lucky that they are offering to pay anything at all.