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I struggled with this one and had to get some ideas from Joe_Mako's approach.
To answer your question about why your lower sample tool provided 255 (all records) is when you add a group by it collects the sample from each unique group from the group by column. In this case, F1 has all unique amounts, so the sample returns all records. Your workflow would work the same even if you excluded the lower sample tool. Link to sample tool documentation: https://help.alteryx.com/2019.3/Sample.htm