I would check the formatting on the lookup field and make sure it’s one of the first four. If not, the lookup thinks your number is a string (probably). I just tried using MIN with a lookup (currency original field) and it worked fine.

Instead of creating a lookup field + formula field, create a rollup field that points to the number field and use this formula:

MIN (values)

What if he doesn’t want the min of all the values, but the min between the number in each record and some constant that’s important to him?

Not sure… that’s a little trickier since Airtable handles arrays relatively strangely. I think people have solved that problem before in other threads. I think it involves some combination of the ARRAY functions.

I also recommend using a rollup field instead of a lookup. Rollup formulas can include references to other fields in the same table as the rollup.