Ok, taxes for dummies:
At the end of the year you make an affidavit about your income tax, before February. It is 35 % of your income minus your expenditures.
Then you have about another 4 months to pay it.
This is your first year.
The second year, you pay it during the year, in advance, because before the end of the year you have no idea of your expendichures.
So, it makes no difference. I wrote almost no difference because there is inflación and to pay today or in 2 months might make a different.
So, when you buy usd, they collect income tax and i don t see any difference between this or paying the income tax through the bank.