The math is very simple. You issue debt in foreign currency, say euros, at a certain effective interest rate say 9%. The new debt goes on your naira denominated balance sheet exchanged at an official rate of N214 per €.
A few weeks later your currency, the naira, loses 40% of its value and now exchanges at N309 per €. Which means the value of the euro denominated debt on your naira denominated balance sheet jumps by about 40%. Your debt servicing costs jump as well. Even though your naira denominated tax revenue does not necessarily jump in tandem.
In short, issuing foreign currency denominated debt when almost everyone expects your domestic currency to soon lose somewhere around 40% of its value is very close to financial suicide. Financial suicide that leads to HIPC status.
Its the reason why foreign investors are dumping naira denominated debt and equity. Its the reason why Nigerians are flocking to foreign currency denominated assets.
The smart thing to do is to wait until after the currency adjustment before selling Eurobonds. Unless, of course, they know an adjustment will happen before then. http://www.bloomberg.com/news/articles/2016-01-22/nigeria-plans-1-billion-eurobond-sale-to-fund-record-deficit