It is permitted to collect the money of a loan during Chol Hamoed. This applies whether the borrower is a Jew or gentile. [This applies even if one is able to collect the loan also after the Moed.] It is, however, forbidden to collect merchandise in exchange for the loan, unless one may not be able to collect the loan after the Moed. If one is owed money for a service or sale of a product, it may be collected during Chol Hamoed if the client/customer was already billed for payment prior to the Moed. If, however, the client/customer was not yet billed for payment, then one may only bill him to collect money during the Moed if one may not be able to do so afterwards. All the above is from the letter of the law, however, the custom today is to collect all debts during the Moed, as it is all considered today as if one may not be able to collect it after the Moed. The Poskim justify this custom.
 Michaber 539:2
 Implication of Michaber ibid; M”B 539:7; Shaar Hatziyon ibid; Kaf Hachaim 539:17; Chol Hamoed Kehilchasa 10:12
 Michaber 539:1
 Michaber 539:3
 Michaber ibid in name of Yeish Mi Sheoser; Tur 539
 Taz 539:1
 Taz 539:1; M”B 539:11; Kaf Hachaim 539:9; Chol Hamoed Kehilchasa 10:11