Is it permissible for him to complete the purchase when he can hear the call to prayer?

No File Exists
Check below answers in case you are looking for other related questions:
Praying Maghrib behind one who is praying ‘Isha’
When one comes to the mosque and finds the last row full

No File Exists
Related Answers:
He followed his imam in Taraweeh to pray ‘Isha’, then he followed him again for the last two rak’ahs
How to pray if not sure imam is traveling or resident
Is there a concession allowing one who is newly married not to pray in congregation?
Praying Maghrib behind one who is praying ‘Isha’
When one comes to the mosque and finds the last row full

Recommended answers for you:
Signs of Allah’s Love for a Person and Attaining It
Meanings of the word fitnah in the Qur'aan
She is sad because she is unmarried and is asking for advice
Places and times where du’aa’ is answered
Who is the Dajjaal