Start with this Du'a
On repaying a loan
بارَكَ اللَّهُ لَكَ فِي أَهْلِكَ وَمَالِكَ، إِنَّمَا جَزَاءُ السَّلَفِ الْحَمْدُ وَالأَدَاءُ
Barakallahu laka fi ahlika wa malik, innama jaza'us-salafil-hamdu wal-ada'.
“May Allah bless you in your family and your wealth. The only reward for a loan is gratitude and repayment.”
Sunan an-Nasa'i 4683 (hasan), Sunan Ibn Majah 2424 (hasan)
The Prophet ﷺ said this to the man who had lent him money, at the time of repaying it.