FIRST FEDERAL DELAWARE AGREEMENT EXTENDED <First Federal Savings Bank> of Delaware said its agreement to negotiate exclusively for its sale with <Oxford Financial Group> has been extended until April 8 from March 18. The company said it is in the final stages of talks with Oxford over the terms of the proposed acquisition. Under a nonbinding letter of intent signed in June 1986, Oxford would pay 11 dlrs per First Federal share, subject to First Federal shareholder approval.