Any permanent installations tenants do with the landlord's permission or consent become part of the property and will belong to the landlord unless and only to the extent that there is a written agreeement to the contrary. In the absence of an agreement stating otherwise, the fence has to stay--if they remove it, you could sue them for its value and for the cost of correcting any damage they do, such as to landscaping, and you are under no obligation to repay or reimburse them for it, unless you had previously agreed to do so. They voluntarily  chose to install a fence for their own benefit on property they did not own; they cannot now, after the fact, seek payment for it or remove it from the property on which they installed it.