Dear Rabbi Kornfeld,
The gemara says:
"As Money is definite, so the equivalent must be definite".
If a man holds a purse full of gold coins (which are legal tender at that time and place) and performs Kidushin with it but neither he nor anyone else knows exactly how many gold coins are in the purse - are the Kidushin invalid because nobody knows the exact value? Or are the considered the "equivalent of money" - and the Kidushin would be invalid according to Rav Yosef?
Although I did not find anyone who speaks about your interesting question, my feeling is that the Mi'ut of Lo Kayitz only applies to Tevu'ah and Kelim, and not to money. In order for us to say Shaveh Kesef k'Kesef we must know the value of those items (Tevu'ah and Kelim). In contrast, for money -- which does not require an assessment (Shuma) even though it may need counting -- we do not need the Ribuy of Shaveh Kesef k'Kesef and therefore it would have a Din of Kesef even if the amount was unknown, since the value does not need a Shuma.
D. Zupnik