If the time limit is 6 months as he say and you have excess panels to cover average winter needs, summer surpluses are lost

More importantly it was left unspecified if the discount given was dollar for dollar electricity. If it's less than that suddenly your ideal, infinite battery becomes less than perfect

But I agree selling to the neighboor is not gonna work - a feasible alternative would be to have the resources pooled for building a larger panels with a larger battery