Upgrading my Lease

[Cassidy Burns]

Hi BP,

I have run into an issue and need some advice.  

My current tenants are breaking their lease and leaving early, December 18th, horrible time to try to rent condo out even in Washington DC.  I told them that they since they broke the lease they would be responsible for the whole month of December and I would be keeping their security deposit.  They responded 3 weeks later stating that my lease never stated they would be responsible for the security if lease is broken, I checked and unfortunately it doesn't.   They have offered to pay the prorated payment for staying through December  18th and forfeiting the security deposit.  

IS this a fair deal or should I respond and try to retrieve the full months rent for December?

ALSO, I now know that I need to upgrade my lease.  How do I go about developing a more detailed lease?  Should I have a different lease for my future units?

Thanks for any help.