Just type some type of letter reminding them of the last day when they have to surrender the keys.

If during the walk through, you determine everything is acceptable, I will turn over my keys and surrender the property to you.

If Tenant(s) abandon or surrender the premises at anytime and leaves personal property there, Agent may dispose of it however Agent chooses, and Tenant shall reimburse Agent for all costs incurred in that regard.Full-disclosure that I have yet to be in a situation where I've needed to enforce that, so TBD how it plays out in practice.

Put that on the current owner...let him deal with the headaches, and surrender a problem/tenant free property to you at closing. 

In many jurisdictions, this would equate to surrendering possession of the unit.Ideally, you want to cross paths with them and have them sign a "return of possession" document.  

I know in Colorado 91 days notice of date to surrender unit  is required to terminate lease if any notice is required by lease.

If one Member wants out, for instance, all they have to do is to surrender their Membership under agreed terms/price etc. 

Never surrender!

The mortgage - note holder must surrender the title to the Dept. of revenue in order to attach the MH to the land nowdays.

The tenant shall at once surrender possessions of the premises to the landlord, and the landlord may without further notice remove all persons and all property from the premises.