Gittin 76 - Divorce for Services
If a husband told his wife, "This is your Get on condition that your serve my father," and the father died, - it is a valid Get. The condition was based on need, and the need was fulfilled.
If a man told his wife, "This is your Get on condition that you nurse my son for two years," and the son died, the Get is void . The husband wanted to harass her, and did not succeed. Other s say that the husband wanted to profit, and that the Get is valid . Practically, we rule stringently, but they should have been more specific!