It depends on the context; the use of «de» is complicated. In this case it is used together with the definite article to express the idea that you have some cash. That is called the "partitive article" -- you're talking about a part of the whole. In English we don't always point that out by using "some", but it is implied.
masculine singular: de + le = du (Je mange du poisson — I am eating (some) fish)
feminine singular: de + la = de la (Je mange de la viande — I am eating (some) meat)
singular before a vowel: de l' (Je mange de l'ananas — I am eating (some) pineapple)
plural: de + les = des (Je mange des légumes -- I am eating (some) vegetables)
So far, relatively simple. But it gets more difficult. When forming negative sentences, the partitive article becomes just «de» for most cases, except as usual before a vowel, we get an elision.
Je ne mange pas de poisson -- I am not eating any fish
Je ne mange pas de viande -- I am not eating any meat
Je ne mange pas d'ananas -- I am not eating any pineapple
Je ne mange pas de légume -- I am not eating any vegetables. (Notice that French also changes to the singular for the object. I don't always see that in colloquial French, and it can depend on whether one's talking about different kinds of things, like for fruits and vegetables, but you are better off learning to use the singular, and later modify it as you become aware of nuances.)
So far so, good. But there are more exceptions. After adverbs of quantity, like «beaucoup» or «peu», it's always «de», affirmative or negative, and even if the object is in plural.
Je mange beaucoup de poisson. -- I am eating a lot of fish
Je mange un peu de viande -- I am eating a bit of meat
Je mange trop d'ananas -- I am eating too much pineapple
Je ne mange pas assez de légumes -- I am not eating enough vegetables
And there's more (don't freak out yet). When used in the sense of belonging, like English "of", it's always «de» (except for elision):
Je boit un verre de vin -- I am drinking a glass of wine
Je boit une tasse d'eau -- I am drinking a cup of water
IMO Duo does not require enough practice for those, and lots of people are confused. I recommend practicing them on your own. Make up lots of sentences. If you are unsure whether you're getting it right, use lang-8.com or italki.com to get corrections from native speakers.
If you want a quick overview of all the uses for the preposition «de», here's a start: http://french.about.com/od/grammar/a/preposition_de.htm