Since you are developing a requirements document and not a technical specification, you should try to be as clear and unambiguous as possible so use common language rather than conventions that maybe only understood by few people.

I would not use: Decimal (13,0).

I'm not sure exactly what your requirement is but I would say something like: The system shall support loan amounts no greater than $999,999 and should have no decimals.

Hope this helps!
- Adrian