According to the Decree of NBU №124 dated and valid from 23.02.2015, were introduced the following restrictions:
1. Pre-payment under the contract for goods / services in amount MORE than 50.000 USD(or equivalent in other currency)  is allowed ONLY  with permission of the NBU.
2. Payments under the contract for goods / services in an amount exceeding US $ 500.000 (or equivalent of another currency) is allowed ONLY  in the form of a letter of credit.
3. It is forbidden to buy foreign currency on credit.
The full text of this Decree:
http://www.bank.gov.ua/doccatalog/document?id=14876621