The Prophet (ﷺ) made this dua when paying back what he borrowed:
بَارَكَ اللَّهُ لَكَ فِي أَهْلِكَ وَمَالِكَ، إِنَّمَا جَزَاءُ السَّلَفِ الْحَمْدُ وَالْأَدَاءُ
Transliteration
BaarakAllahu laka fee 'ahlika wa maalika, 'innamaa jazaa'us-salafil-hamdu wal'adaa'
Translation
May Allah bless you in your family and your wealth, surely the reward for a loan is praise and returning (what was borrowed).
Say Alhamdulillah when debt is gone. You can also say Allahumma ikfini bihalalika. These prayers thank Allah and ask for good money and help.
Say Jazakallahu Khayran first. Then make dua: May Allah bless you and give you more money. This prayer for family and wealth shows real thanks.
Thank Allah when you pay. Thank the person too. Make dua for both of you. Pay quickly, show good feelings, and pray for blessings - that's Islam's way.
No fixed prayer exists. But say Allahumma inni a'udhu bika min al-ma'tham wal-maghram for safety. Alhamdulillah works as a thank you prayer too.
Calm comes to the heart. An Islamic task is done. Trust grows, daily prayers get easier, and ties with others become stronger through thanks and blessings.