They took 7 months to install the panels. I'm sure times during all that the OP thought if they forgot about him, called them to ask for a installation date, etc.

As a way to curb any thought about undue wait time, the company made a contract guaranteeing installation between 6 month after signing. They didn't comply and the loan lapsed. Now the company needed to negotiate a new contract with OP or go for the legal route. There is nothing unethical about this.

The company had everything to lose going the the legal option, with all the long terms procedures, paying money for a pay back that may never come. They are probably grateful that OP just asked for a discount instead.