1. The Bank is right in asking for the link document,
2. In fact the Bank should ask for the original link document,
3. It is so asked because, your vendor may mortgage his property by depositing his original title deed of the said property with a Bank while taking loan,
4. In such case, a charge on the said property is created in favour of the mortgagee Bank,
5. After mortgaging the said property, which is not registered, he can execute and register a sale deed of the same property to someone in which case the said sale of the already mortgaged property will be invalid,
6. You should collect the original link deed in your own interest.