Primary Dua for Debt Relief
اللَّهُمَّ اكْفِنِي بِحَلَالِكَ عَنْ حَرَامِكَ، وَأَغْنِنِي بِفَضْلِكَ عَمَّنْ سِوَاكَ
Meaning: O Allah, suffice me with Your halal from Your haram, and enrich me by Your grace from all others besides You.
Tirmidhi 3563 (Hasan)
The Prophet's Dua for Debt
اللَّهُمَّ إِنِّي أَعُوذُ بِكَ مِنَ الْهَمِّ وَالْحَزَنِ، وَأَعُوذُ بِكَ مِنَ الْعَجْزِ وَالْكَسَلِ، وَأَعُوذُ بِكَ مِنَ الْجُبْنِ وَالْبُخْلِ، وَأَعُوذُ بِكَ مِنْ غَلَبَةِ الدَّيْنِ وَقَهْرِ الرِّجَالِ
This is the same as the Dua for Anxiety, which specifically includes "being heavily in debt" as one of the seven things to seek refuge from.