بسم الله الرحمن الرحيم
Apparently, it is not the deal of shirkah (partnership) rather it is to derive benefit by giving loan and it is not lawful; hence it shall not be lawful for you to take fifteen thousand extra.
كل قرض جر منفعة فهو ربا (مصنف بن أبي شيبه، رقم: 20690)
[Translation: Every debt that yields profits, is a usury.]
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.