Commentary for Kiddushin 120:16

He must show her his won money for she did not agree to simply see money belonging to someone else.

The mishnah again seems to teach something obvious—if he just shows her money on the table she is not betrothed.
The Talmud resolves this by saying that even if this is money someone gave him to invest, she is not betrothed. The money needs to be his, not just given to him to use as an investment.