..the borrower (you) can just repay the loan by using "refund" function, and send the interests with another transfer. ..
Nope. For that to work the amount that was transferred to his account must still be there. As far as I know, you can't "refund" from a different source - that would be similar to a simple transaction.
To the OP. If you need an explainer video or any kind of video - I'm here offering my services
I'll do you a solid and give you a free work provided the work does not take more than 24 hours for me to animate + render. So shoot me a PM with details.Offer redacted. OP wanted a much more complex animation that would not have been possible to do within 24 hours. I estimated the value of the video he wanted to create at around $300 for a week-long work. And when I wanted to put my branding/logo on one corner of the video OP didn't come to an agreement.