Just point out the maths to them, if a property has been empty for 2 months at an asking rent of £600/m, they have already lost out on £1200, if it's empty for another month, there's another £600, so offer £50 less and state that it's only a months rent over 12 months but guarantees a tenant for that length of time and no more loss, it will more likely work if the property has been on the market for quite a while though, i got £25 / month off a place once simply by saying i'll take it if he does it for x amount, they acepted straight away.