Can I, the landlord, keep a months security since my tentant only gave me 10 days notice about not renewing but said he would renew a few wks ago.

Note, the lease is a realtor's lease and doesn't specifically say anything about a time frame to notify the landlord unless there are changes to the lease, which there was not. It just seems to me that to expect someone to find a suitable renter in 10 days is unreasonable and therefore, the only recourse might be to hold back on a month's worth of security (out of the month and half given), if a new tenant is not found in time. Kindly please advise, thanks.