Singer Acquisition Co. has extended its tender offer for all outstanding common shares of Singer Co. at $50 per share in cash until 5 p.m. EST Monday . . . Lotus Development Corp. dropped its lawsuit against Rossin, Greenberg, Seronick & Hill, a Boston advertising agency, after the agency agreed to permanent protection of Lotus' confidential information and not to solicit or to perform work for entities manufacturing or selling microcomputer software products for 10 months without Lotus' consent . . . Washington Post Co. said it will record an after-tax gain of about $115.5 million, or about $9 a share, in the first quarter of 1988 on the sale of its Florida cellular telephone operations to McCaw Communication Cos.