Volcano: If you're looking for a literal translation of a title that already exists, then "head" is odd, but okay. However, if you're just writing up a new title, a "head department" is an unusual term, that I think doesn't really make good sense. A department usually has a "head' or leader, but it's unclear to what a "head department" could mean.

"External transactions" is a much broader term than "foreign transactions." So, it depends on what you have in mind. If credit and, specifically, foreign transactions are the essential or predominant domains of the department, I'd go with "foreign transactions." If you look up the definition of "external transactions," you will find that this refers to ANY transaction between two separate entities. So, if foreign transactions are just one among many kinds of external transactions handled by the department, I'd go with "external transactions." Hope this helps!