I assume the assessment was due or past due.
In most cases assessments for local benefits are not tax deductible. Please follow this link and review IRS Pub 530 for a discussion of deductible and non-deductible special assessments. https://www.irs.gov/publications/p530/ar02.html#en_US_2016_publink100011838
If the assessment is not deductible as a form of property tax, the assessment you paid gets added to your basis in the property for purposes of calculating your gain/loss on the sale.