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

If that is the adhaan for Jumu‘ah prayer that comes after the khateeb has ascended the minbar, then it is haraam to buy and sell at that time, because Allah says (interpretation of the meaning): “O you who believe (Muslims)! When the call is proclaimed for the Salaah (prayer) on Friday (Jumu‘ah prayer), come to the remembrance of Allaah [Jumu‘ah religious talk (Khutbah) and Salaah (prayer)] and leave off business (and every other thing). That is better for you if you did but know!” [al-Jumu‘ah 62:9]. 

The words “O you who believe (Muslims)! When the call is proclaimed for the Salaah (prayer) on Friday (Jumu‘ah prayer), come to the remembrance of Allaah [Jumu‘ah religious talk (Khutbah) and Salaah (prayer)] and leave off business” mean: stop dealing with trade and come and listen to the khutbah and perform Jumu‘ah prayer in the mosque with the imam. This means that it is haraam to buy and sell after the second adhaan which comes when the khateeb sits on the minbar, until the prayer ends, unless there is a case of necessity which calls for buying or selling, such as buying water for the purpose of purification or a garment to cover one’s ‘awrah for prayer. End quote. 

Fataawa al-Lajnah al-Daa’imah, 13/101-102 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

Buying and selling after the second call to Jumu‘ah is haraam and also invalid. Based on that, the transaction is invalid, so it is not permissible for the purchaser to dispose of what he has bought, because he has not taken possession of it, and it is not permissible for the seller to dispose of the money paid, because he has not taken possession of it. This is a serious matter, because some people may buy or sell after the second call to Jumu‘ah, then take things on the assumption that they now belong to them. End quote. 

Al-Sharh al-Mumti‘, 8/52. 

So it is not permissible to buy or sell after the second adhaan for Jumu‘ah, because what is required is to hasten to the mosque in order to hear the khutbah and pray, and buying and selling distracts from that. 

But if that happens with regard to all the other prayers (Fajr, Zuhr, ‘Asr, Maghrib and ‘Isha’), there is nothing wrong with it, so long as it does not distract one from praying in congregation and cause one to miss it. 

Allah says (interpretation of the meaning): “Men whom neither trade nor sale (business) diverts from the remembrance of Allaah (with heart and tongue) nor from performing As‑Salât (Iqâmat‑as‑Salât) nor from giving the Zakât. They fear a Day when hearts and eyes will be overturned (out of the horror of the torment of the Day of Resurrection)” [al-Noor 24:37]. 

Those are men who, even if they do business and sell and buy, there is nothing haraam in that, but it should not distract them and make them give precedence to that and prefer it over the remembrance of Allah, establishing prayer and giving zakaah; rather they should make obedience to Allah and worship of Him their ultimate aim, so whatever keeps them from that, they should reject it.

End quote from Tafseer al-Sa‘di, p. 569 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: Is it haraam to buy and sell during the adhaan or after it? What about on Fridays? 

He replied: 

Allah says (interpretation of the meaning): “O you who believe (Muslims)! When the call is proclaimed for the Salaah (prayer) on Friday (Jumu‘ah prayer)…” [al-Jumu‘ah 62:9], so it is not permissible to buy and sell or to rent or do any other kind of transaction; rather people should focus on Jumu‘ah prayer and hasten to pray Jumu‘ah, and not let anything else distract them. 

With regard to other prayers, they may or may not come under the same rulings as Jumu‘ah, but to be on the safe side, one should not do anything after the call to prayer, after the adhaan for Zuhr, ‘Asr or Maghrib, because that may distract him from praying in congregation. So it is better to avoid that, unless it is some minor matter that will not distract him, so perhaps there is nothing wrong with it, because Allah, may He be glorified and exalted, only spoke about the issue of Jumu‘ah, because it is very important and it is obligatory to attend it, and one misses out by missing it. So it is a serious matter and is a weekly obligation. 

To sum up: Jumu‘ah cannot be compared with other prayers, but if he avoids such situations so that he will not be distracted from praying in congregation, that is better. 

Whatever the case, if his buying and selling may distract him from performing prayer in congregation, then it is haraam, but in some cases the prayer may be delayed, if the Imam is late, so a person may buy or sell something on his way, and it may not affect his attending the prayer. But if he avoids that and adheres to what applies to Jumu‘ah, that will be more on the safe side, so it is like Jumu‘ah with regard to precautions. End quote. 

From Fataawa Noor ‘ala al-Darb

And Allah knows best.