Security Deposit/Tenant backing out

[Shane Moynihan]

So I have a bit of a dilemma, I have a tenant who signed a 6 month lease that runs Dec 15 - June 15. They signed the lease and has given me $750 of of the $1000 due for the security deposit. Now they are claiming that they do not think they will be able to afford the rent anymore due to circumstances. They have already been late paying the security deposit and now asking if I can cancel the lease and have them get back their $750. 

Am I allowed to keep this as they have wasted almost a month of my time and not getting my money for rent? Or should I do like 50/50 or do I just give it back? 

Thank you in advance!