Sentence:  On May 23, Levin drew up a suggested deal with CBS, whereby they would put up $300,000 plus an <WSD>overcall</WSD> of 20 percent ($60,000); CBS would get the rights to televise the show, while the stage version would get television and radio publicity, with the possibility of television coverage of the opening night. 
Meaning of the word marked with <WSD> tags:  An additional contribution required of investors beyond the initial investment, should unforeseen expenses arise.