It's nice if your language has support for monetary amounts, but usually you end up using multiple languages that interact with the same database model, and you still end up using a built in numeric datatype in your RDBMS of choice.
Three additional 'mistakes' to prevent when dealing with money representations:
1. The definition of a currency might change. For example, some years ago Iceland decided to change the exponent of ISK from 2 to 0. Currencies have different versions.
2. As a FinTech you probably have integrations with many third parties, they don't change their exponents for a currency at the same time. Keep track of what third parties think the correct exponent is at any point in time, and convert between your representation and their representation. Otherwise, you'll have interesting incidents (e.g. transferring 100x the intended amount of ISK).
3. At first you think that counting minor units as an integer is enough, and then you need to start accounting for fractions of cents because sales people sold something for a fee of $0.0042 per transaction. If your code rounds all these fees to $0.00 you don't make any money.
1, 2 - Yeah, there's always a sanity checking and conversion layer around the 3rd party. Currencies do indeed sometimes have 2 versions in play there.
3 - Indeed, the currency type that I referenced "is appropriate for financial calculations that require large numbers of significant integral and fractional digits and no round-off errors"
Three additional 'mistakes' to prevent when dealing with money representations:
1. The definition of a currency might change. For example, some years ago Iceland decided to change the exponent of ISK from 2 to 0. Currencies have different versions.
2. As a FinTech you probably have integrations with many third parties, they don't change their exponents for a currency at the same time. Keep track of what third parties think the correct exponent is at any point in time, and convert between your representation and their representation. Otherwise, you'll have interesting incidents (e.g. transferring 100x the intended amount of ISK).
3. At first you think that counting minor units as an integer is enough, and then you need to start accounting for fractions of cents because sales people sold something for a fee of $0.0042 per transaction. If your code rounds all these fees to $0.00 you don't make any money.