How must a debt collector respond to a request to validate a debt?

A law firm contacted me via registered mail claiming I must pay them for money owed my condo home owners association. I wrote them and asked them to validate the debt and can show they received my letter. I never received a response. They continued to pursue collections and put a lien on my condo about a month after they received my request for debt validation. I contacted them about this and they said that they sent a valiation of debt via mail, but not registered mail etc. They can not then prove that I received validation of the debt. What can I do? Can I sue based on FDCPA violation? Now they want a lot more fees/money then they did in the first contact. Shouldn't they have stopped collection until they gave me proof I owed the debt and they were authorized to collect it? If the debt is valid I will pay it. But I don't think I should pay more because they failed to validate the debt properly.