Translation:Soon we will save up money, but not tonight!
"Soon will we save money, but not tonight" should be accepted. It's a bit poetic, but this sentence is in a more poetic mood than most.
And it's entirely grammatical with exactly the same meaning as the currently accepted answer. (It has the additional advantage of using a similar word order to the German.)
'Economize' is not quite the same as 'save', though they are two sides of the same coin. First of all, it does not take an object, so you don't 'economize money', you just 'economize', as in 'Our income has gone down again this month, so we need to economize.' The focus is on spending less because the money simply isn't there, as opposed to 'saving money ' to accumulate a large sum e.g. for a holiday abroad. (Native speaker)
Two years later, hah
Giving my two cents as a learner, it seems that "sondern" is meant more as a kind of "rather", or "but instead". Example: "Ich habe nicht zwei, sondern drei?", or "I have not one, but two?". Whereas "aber" seems to be more of just a contradictory "but", same with "doch".
I could easily be wrong, but this is my understanding of it.
I don't believe it. In the same session 'Soon we shall save money but not tonight' was also marked wrong. This is another correct translation. I don't know whether Duolingo doesn't like the 'shall' instead of 'will' or the fact that 'up' is missing. Either way it's perfectly good English. I can't speak for usage across the pond, but in British English, one tends to 'save money' rather than 'save up' money. But you do 'save up' to buy a new dishwasher, for example.