Sorry, no. It just increases the price of the house to you. The credit is only available to the person who installed it and the credit is effective on the date that the solar installation was placed into service (turned on).
The seller is still eligible for the credit even though the lien has not been paid. Because the loan is in their name and must eventually paid back by them, the seller is eligible for the credit.
You should take this into account when you negotiate a price.
