On Kidushin 47A: What is The Power of the Remaining Peruta?
When a man says to a woman: "Be thou betrothed to me with the deposit which I have in your possession" - and she agrre, ans when she goes to take the deposit she finds that it has been stolen or lost - if the value of a peruta is left the betrothal is valid, and if nothing is left - she is not betrothed"
Why is the betrothal valid? Assume the deposit was for the whole wealth of the man, amounting to 1,000,000 perutah, and only one perutah was left. Indeed she does take possession of the deposit, or rather what is left of it, which in itself is of sufficient value - but that was NOT the intention of the man: he wanted to betroth this woman with the whole sum of his deposit, with the intent that she be a rich woman! Now they are both paupers! surely this is Mekah Tauth of the first magnitude!
The Rosh writes that it is only valid when the woman did not know the original value of the Pikadon. The Rashba writes that since the man did not specify the value of the Pikadon nor ask if it still exists it is as if he said "with whatever is left."
D. Zupnik