Is supplicating for something difficult to attain but not impossible, such as "O Allah, grant me fifty billion [currency unit] within this week," considered an transgression in supplication?

Allah is not incapable of anything, but it is considered an transgression in supplication for a servant to ask for something that is ordinarily impossible, or to ask Allah for something that is not in accordance with His established cosmic laws. Obtaining a very large sum of money in a short time is not part of Allah's established cosmic laws, and this may be considered an transgression in supplication. A Muslim should not have such an aspiration; rather, he should supplicate to his Lord, constrained by what is good.