Most landlords are cash basis taxpayers, and I suspect you are one also. The answer is no, unearned rent is not an allowed expense for a cash basis taxpayer.
Think about it this way. I bought a lottery ticket last month when the lottery jackpot was $240MM. Needless to say, I did not win. If I apply your argument, I should be able to take a tax loss for the $240MM jackpot I did not win.
One other question. Does the income that I received from my rentals added to my salary income? And if so, Can I subtract all of my expenses (allowable deductions) to my salary to reduce the taxable income as much as possible?
Rental property operation is a passive income activity with all of your rental income and expenses reported on Schedule E (1040) of your tax return. With the exception of depreciation, you can only expense something related to your rental activity if you actually pay for it out of pocket. A vacancy may deny you income, but it did not actually take money out of your pocket – no deduction.