What is the relationship between dana and tax? Is dana (or has it ever been) considered taxable in some countries?
Do you mind an unreferenced answer?

I think the answer is "generally, no"; but that you're asking after any examples, however rare, where the answer has been yes.

Also I guess that gifts of wealth are usually made to an institution e.g. a monastery (as opposed to less-easily-taxable gifts of food and other requisites, given to a monk).


Historically where a country had Muslim conquerors (e.g. India, Afghanistan) there was sometimes a poll tax (a head-tax) on non-Muslims.
There were a few short periods, a thousand years ago or so, where Chinese emperors were troubled by the tax-exempt (and war-exempt) status of Buddhists, and tried to regulate or tax or suppress it
In the Soviet Union under Stalin (e.g. in 1928), they decided that Buddhism (even though atheist) was anti-revolutionary, and taxed monasteries
In Thailand a few years ago there was a proposal to tax monasteries (however I think that was not implemented)
In 1985-1987 Kyoto tried to add a tax to the price of admission tickets which the Buddhist temples sold to tourists