2,704. The Witness to a Man's Death and an Oath of Testimony
Hilchos Shevuos 10:11
If a woman imposes an oath on a lone witness to testify that her husband died and he denies having information, he is liable for an oath of testimony. This is because, had he testified, she would have been able to remarry and she would have received the value of her kesubah.
Hilchos Shevuos 10:12
The previous halacha applies when the woman could collect the value of her kesubah from movable goods; if she can only collect her kesubah from real estate, then the witness isn’t liable for an oath of testimony. This is also the case when there are two witnesses because when one imposes an oath regarding real estate, one simply isn’t liable for an oath of testimony.