The purchaser at the HOA foreclosure, provided the foreclosure was properly performed and after issuance of the certificate of title in their name, should now own the subject property subject to any superior interests, many times the first mortgage. The pending bank foreclosure does not effect this right in relation to your ability to remain in the property.
That being said, the owner is offering the option of staying in the property at an undisclosed rental amount. This is not something they are required to do but rather something they offering in the alternative to removal from the premises. The time limit is not appropriate however. The appropriate action the purchaser must take is called a writ of possession. Such action occurs within the Court System and culminates with the Sheriff providing you notice and a time frame to vacate.
If you are a tenant rather than the owner pursuant to the qualifications of the Federal Protecting Tenants in Foreclosure Act, additional time is mandated by law.