Translation:We save money to open a restaurant.
'Saving' changes in translation depending on what part of speech it is. In "We are saving...", 'saving' is the present progressive form of the verb 'to save', so it would be translated literally as "Nós estamos poupando..." But the gerund 'saving' can also be used as an adjective or a noun, though it is usually used in the plural 'savings', as in "I put the money in my savings account" or "The total of our savings is $100." When it is used as a noun, its translation is 'poupança', just as your dictionary told you.
Poupamos is the plural "we save", but present simple doesn't work in English as it suggests that's our function in life or sole activity; it's what we do, like the sun shines, Present continuous is the only translation that works. I offer English classes at outrageous prices, but can often be beaten down to something more equitable.. :) perhaps you should try a different dictionary; nós estamos poupando sounds like a better translation..