| Presented By Rav Yosef Greenwald, Rav of k'hal Dexter Park
Answer: When the Gemara speaks about cases of "nehene", it typically is speaking about when one person takes something from someone else. In this case, the homeowner isn't taking the fan from anyone else; rather, he had it foisted on him. This is, therefore, not a case of nehene.
Instead, this would be a case of "yored l'sadeh chaveiro". This refers to a Gemara in Bava Metziah where someone goes into his friend's field and plants a tree or does any form of work that upgrades the property, without being asked to do so. The halacha in such a case is that if the homeowner wants to keep the upgrade, he would have to pay a certain amount, but he would not have to pay the full value of the service. Since he did not contract the worker to work in his field, he doesn't have to pay for the job as if he had. Instead, he only has to pay the amount that his property went up in value as a result of the upgrade, which is a relatively small amount.
Additionally, he cannot be forced to accept the addition to his field, so he can tell the worker to take back the upgrade he made to his property. In this case, that would mean that he can theoretically tell the worker to take back the fan and fix the roof to be the way it was before. If he makes this request, it may not be worthwhile for the roofer to do all that extra work and he may just decide to leave it there for free. | 
