You are allowed to receive income tax deductions for such donations, including zakah as well as other charitable contributions; but you should keep track of the amount you receive for zakah separately from other charitable contributions. The portion of tax deductions that you receive for your zakah, is best given out to the poor and needy; although it may be allowed to give a portion of the same towards mosque, if it provides educational services such as school, etc. As for deductions you receive for your other charitable contributions, you may give them towards the mosque or any other good cause you may deem appropriate.

It is important, as you have realized, not to make use of such deductions that we receive in lieu of zakah contributions, for our own personal benefit. As for other charitable contributions, we may be allowed to use them, if while giving such donations, we had the intention to do so. It is, however, best in cases, when you have a doubt, to do what is closer to piety: By giving them away as charity.