Nothing But the Truth (1941)

Stockbroker T.T. Ralston has promised his niece Gwen to double it if she can raise $20,000 for charity. But he connives so those she asks refuse to give her more than the $10,000 she's already raised. Gwen secretly gives T.T.'s new partner Steve Bennett the $10,000 and asks him to double it. Later in a discussion with T.T., business partner Dick and client Van, about truth in business Steve bets the $10,000 that he can tell the truth for 24 hours. Having already accepted an invitation for a weekend on T.T.'s houseboat, Steve is subjected to 24 hours of attempts to get him to lie or divulge the bet, which would lose him (and Gwen) the $10,000.