(1) Let's say your charitable contribution of $100 resulted in a deduction of $1 from your tax bill. Would your maaser contribution from that hundred be counted as $99 total or $100 total?
(2) Let's say your maaser above only counted as $99--but you owe $100 in maaser. How do you pay that, in light of the fact that the more maaser you pay, the lower your tax bill gets, and the higher your maaserable income? (see here)
(I'm more interested in whether this is addressed in halacha than whether it is mathematically or practically feasible, since I'm pretty sure it is both of those.)
Even more fun maaser and tax math here!