The Terrier, who holds both Nigerian and Danish passports, was born in Denmark but is of Nigerian descent.
The Dane has represented his country of birth at both Under-19 and Under-21 level - winning four caps for the latter.
Billing is yet to represent either nation at senior level. Having already represented Denmark this puts the Danish Football Federation (or Danish Ballgames Union) in the pole position to secure his services.
The Nigerian Football Federation have a record of pursuading those of Nigerian decent to play for them instead of their country of birth, the most recent being Nnamdi Ofoborh from AFC Bournemouth.
Ofoborh was born in England but was convinced to play for the three times African champions by Amaju Pinnick the President of the Nigerian Federation. Ofoborah made his debut for the under-19 squad towards the end of March.
Nigeria or Denmark
Billing could well follow suit and make a senior appearance for Nigeria ahead of Denmark. Although both teams have qualified for the World Cup in Russia, the strength and depth in the Danish side could see Billing on the sidelines in the senior squad if selected.
If the choice comes between senior level football representing your country at a World Cup, or under-21 level football, there really should be no contest. The Dane is very likely to take the opportunity for senior football, which would mean an allegiance switch, however, with dual nationality that would not cause any major problems for the Terrier.