Best way to select currency

We're developing a feature for our system that allows to relate a certain cost to an hour of working time under a category. So the user will be able to set that a working hour of the category 'IT Support' may costs $1000.

The issue here is that I don't know which is the best way to let the user define the currency they want to use. The cost of the working hours are going to be shown in Excel reports and in a register in a history view.

It has occurred to me that maybe it would be fine if we let the user write the symbol/abbreviation they want to use and then they will have to decide if they want it as a prefix o a suffix. This means we wouldn't use a currency selector.

We have users all around the world.

We have the same problem with thousands and decimal separators. Should we let the user write which separator do they wnat for thousands and wich one for decimals?