Tenant gave move -out notice while 1 month + 10 days remaining

[A G.]

Hi All - Wanted to get some advice on a situation where a tenant (who is also real estate agent), have a notice of move-out (Dec 20th) which we only received today (post marked dec 5th), requesting that as per RCW 59.18.900, exercising the right to vacate the property and no liable to early lease break. He is also requesting that remaining dec rent and Jan rent (last month rent paid along with security deposit) be returned. He insist that we should meet on 20th to collect the keys so it would sent by mail. What I don’t understand is which clause he is referring to with this notice when he signed a 12 month lease? We have full rent (Jan 2018) and security deposit (which we intend to return), but not sure what would be next steps if we don’t return any rent (which we don’t think we should). What do you think he is planning for? Just playing hard referring to legal clause hoping we may get afraid and return some rent? Just for context, in beginning to nov, we told him that we would be increasing the rent that he complained and then stopped communicating for almost a month. Then one sudden day called and said he would be vacating the property and is not obligated to pay rent till end of month. Thoughts?