You need to create a paper trail documenting that the $100K is a loan, with appropriate documentation such as a loan agreement, promissory note, interest rate being charged, loan repayment terms etc. You and the borrower should each have a fully executed copy of all this paperwork. You should also send the money via regular banking channels (including various money transfer services such as Transferwise or Xoom) and not through hawala channels.
With loans to relatives, and especially with relatives who need money in an emergency, these details can be left till after the immediate transfusion of cash has been made, and there can be sticking points such as you might not want to say you are charging interest (whether paid monthly or quarterly or annually or as a lump-sum along with the principal, as per the loan agreement) on a loan to immediate family members, but this can be handled by you gifting the interest to the borrower each year (and if you prefer, the borrower sending the money back you you as interest). Since the annual interest on $100K should be far less than the annual exemption for gift tax, no Form 709 need be filed. Be that as it may, the annual imputed interest due is taxable income to you for that year, and should be reported as interest income on each year's tax return. When the loan is paid off, you don't need to do any reporting; your bank will report that you received $100K or $100K+, and you have the paperwork to prove that it is merely repayment of a loan; it need not be reported on a tax return, and no tax is due.
Be aware that in some countries, repayment of such loans is allowed only in the currency of the foreign country and only into the lender's bank account in that country. Thus, if your relative is in such a country, you will need to establish such an account if you don't have it already, and you will bear the exchange rate risk. That US$100K might have become a loan of MN$300k, and be paid back as MN$300K, and when you bring the money back into the US, you might be getting back (say) only US$90K instead of the US$100K that you loaned because the exchange rate between US$ and MN$ changed in the interim.
