Student or Learner
Do we say "2 dollars have been applied or has been applied"?
It depends on whether you think of them as individual dollars or as a lump sum.
Of the five dollars you gave me, two have been applied to your debt and three went to buy us these snacks.
[A sum of] two dollars has been applied to the debt you owe me.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.