Reading M&F, I came across this phrase, but isn’t it wrong? Shouldn’t poena be in the sg. instead, since it is being translated as “paying the penalty”?
How someone chooses to translate a phrase into idiomatic English has no effect on what is idiomatic Latin. As you’ll see in the entry for poena any good dictionary, the Latin expression is poenas dare. You may think it sounds strange – because it doesn’t match what English would do – but it’s what Latin does.
As a Swedish speaker, I could object to an expression such as “change trains”, because in Swedish, the expression is “change train”. But English is English. And Latin is Latin!