Say that I borrow 100 pounds from somebody and agree with him that the debt should be repayed in Pakistani rupees. If the 100 pounds corresponds to 14 945 rupees, is it permissible to agree with the lender to pay back 15 000 rupees as this figure is easier to remember?
Or will it become riba?

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.

One of the principles in Islamic Finance is:
“Consideration is given to the essence of a transaction and not the phraseology.”[1]

The implications of the above cited maxim can be gauged from the following transaction: ABC gifts an item to XYZ with a conditional clause of XYZ gifting another item to ABC.   Despite the terminologies of gifting present, the substance and essence of the deal is of a barter trade.  The laws of Hibah (gifting) will not apply; instead, the laws of barter trading and sales will apply.
Similarly, in the transaction under question, despite the phraseology of Qardh (loaning) and borrowing being used, the transaction will be regarded as a foreign currency exchange transaction and not a Qardh (loan).  Thus, the laws of currency exchange will dictate the transaction.

When two currencies are exchanged with a deferral, the following laws must be adhered to:

1)One transacting party must acquire possession of the counter currency at the time of striking the deal.[2]
2)The currencies may only be sold at the current market rate; otherwise it will become a means of effecting interest-bearing transactions.[3]

Thus, it will not be permissible to give 15000 PKR if this amount does not equate to the current market rate of 100 GBP at the time of transaction.  You will have to give him 14945 PKR which you state was the corresponding value.
In future, any such exchange of currencies should be treated as transactions of currencies with the above laws governing the transaction.
And Allah Ta’ālā Knows Best

Mufti Faraz Adam,

[1]  الْعِبْرَةُ فِي الْعُقُودِ لِلْمَقَاصِدِ وَالْمَعَانِي لَا لِلْأَلْفَاظِ وَالْمَبَانِي (المجلة : المادة 3)

[2]  هذه الأوراق قد أصبحت اليوم أثمانا عرفية بنفسها فدفعها دفع للمال أو للثمن وليس حوالة للدين فتتأدى بأدائها الزكاة ويجوز شراء الذهب والفضة بها (بحوث في قضايا فقهية معاصرة، أحكام الأوراق النقدية: ص 145؛ كراتشي)

النقود الورقية في حكم الفلوس سواء بسواء (بحوث في قضايا فقهية معاصرة ص 160)

(أحسن الفتاوى، كتاب البيوع: 6/517؛ سعيد)

[3]  ولكن جواز النسيئة في تبادل العملات المختلفة يمكن أن يتخذ حيلة لأكل الربا، فمثلا إذا أراد المقرض أن يطالب بعشر ربيات على المئة المقرضة، فإنه يبيع مئة ربية نسيئة بمقدار من الدولارات التي تساوي مئة وعشر ربيات. وسدا لهذا الباب، فإنه ينبغي أن يقيد جواز النسيئة في بيع العملات أن يقع على سعر السوق السائد عند العقد (بحوث في قضايا فقهية معاصرة ص 170)