"I need money to pay for this cup of tea."
Translation:Necesito dinero para pagar esta taza de té.
'pagar' is 'to pay (for)' and in this case 'para' means 'in order to'. So 'para pagar' is '(in order) to pay for'. The other way around is not grammatical.
If there is a 'to' in sentence that you can exchange with 'in order to' without changing meaning, you have to translate it with 'para'.
'Necesito dinero para pagar por esta taza de té' was marked as wrong. Why is it wrong to use 'por' here? I guess it might be redundant, but is it wrong?
I'm quite pleased with myself that I managed to compose this sentence correctly. (in order) to pay for'' makes it tempting to go 'para pagar por/para' and then there's the question if you need a stray 'a' here or there.