Past research offers conflicting findings on whether sadness-evoking charity appeals help solicit a donation. To reconcile these findings, we introduce prospective donors' regulatory focus as a moderator for understanding when and why sadness appeals motivate or demotivate giving. Specifically, we propose that the sense of helplessness or loss of control associated with sadness appeals increases donors' sensitivity to advertiser's manipulative persuasion tactics, as those tactics can threaten donors' control over their donation decision. As a result, sadness appeals are more likely to activate persuasion knowledge among prevention- (vs. promotion-) oriented donors who tend to be vigilant against manipulative persuasion attempts. Across six main studies and two supplementary studies, we find that a prevention (vs. promotion) focus discourages charitable giving when it is solicited using a sadness appeal, whereas regulatory focus does not affect the giving when other emotion appeals (e.g., happiness appeal or guilt appeal) are used. We find that a prevention (vs. promotion) focus demotivates donation solicited by a sadness appeal because it activates persuasion knowledge that evaluates solicitor's motive behind the sadness appeal, resulting in increased skepticism, dampened feelings of sympathy, and consequently, reduced charitable giving. However, when persuasion knowledge is deactivated (e.g., when donors' cognitive capacity is constrained or the soliciting charity has a reliable reputation), regulatory focus no longer affects donor skepticism, sympathy, and charitable giving, even when a sadness appeal is used to call for donation.