Prophet (SAW)'s Prayers

Loving someone or when one helps
When you love someone make it known by saying:
أَحَبَّكَ الَّذِي أَحْبَبْتَنِي لَهُ
"May Allah love you for whom you have loved me".

(Abu Dawood)
Dua for the one who has aided you
جَزَاكَ اللَّهُ خَيْرَاً
"May Allah grant you (in return) a good reward".

Dua when the debtor pays his debt
أَوْفَيْتَنِي أَوْفَى اللَّهُ بِكَ
"You have paid my debt, may Allah give you plentiful (in this world and in the Askhirat)"

(Hisnul Hasin)