Previous rather than MINX

Here is a column expression that does that.  Note that in your example pbix, I had to change the data type of the Total In column to number.

Diff From Prev =
VAR thissale = Theory[Total IN]
VAR thisdate = Theory[Date Offer Accepted]
VAR prevdate =
    CALCULATE (
        MAX ( Theory[Date Offer Accepted] ),
        ALLEXCEPT (
            Theory,
            Theory[Property Name]
        ),
        Theory[Date Offer Accepted] < thisdate
    )
VAR prevsale =
    CALCULATE (
        SUM ( Theory[Total IN] ),
        ALLEXCEPT (
            Theory,
            Theory[Property Name]
        ),
        Theory[Date Offer Accepted] = prevdate
    )
RETURN
    IF (
        ISBLANK ( prevsale ),
        BLANK (),
        thissale - prevsale
    )

Regards,

Pat