Have you signed renewal contract with your tenants? If yes, the contract should clearly said that you received the two months deposit, so you should take the $600 difference. This can also use to cover for future rents that the tenant owe you and anything that need to be repaired when they move out. When the tenants renew contracts for several times and you increase the rents again and again, the difference will become more significant. So better to ask for the difference each time on renewal than ask them at a large lump sum when it rolls bigger and bigger.