Eclipse, an open source software project, acknowledges its donors by presenting donation badges in its issue tracking system Bugzilla. However, the rewarding effect of this strategy is currently unknown. We applied a framework of causal inference to investigate relative promptness of developer response to bug reports with donation badges compared with bug reports without the badges, and estimated that donation badges decreases developer response time by a median time of about two hours. The appearance of donation badges is appealing for both donors and organizers because of its practical, rewarding and yet inexpensive effect.