If you want to take back the deposit include it as part of your deed of surrender
I have posted previously on how to do this Tenant asking for tenancy end
Once a deed of surrender is complete you cannot claim anything so terms must be dealt with and completed before signing
I usually ask for a months rent and a deposit to surrender an AST early on
You are more informed of your situation and your tenant so you decide if you want to include terns into the lease
I would pass on the legal charge of the lease to the tenant, though!)