Custom Tooltip on Matrix Based on Measure

Hi , cristianpark 

Try steps as below:

1.create a table contain a column as below

2. Then create measures "value" (to instead the field "fixed" and "available") ,and put it into the value of matrix 

Value = SWITCH(SELECTEDVALUE('Column'[Column]),"Fixed",AVERAGE('Table'[Fixed]),"Available",AVERAGE('Table'[Available]))

3.create page3 as  tooltips page ( make sure the type of tooltip is report page)

Put  measure "tooltip text" into a card visual in page3

Tooltip Text = 
VAR selectedType=SELECTEDVALUE('Column'[Column], BLANK())
RETURN
IF(selectedType<>BLANK(),selectedType, "Nothing")&" Measure: "&[Value]

4. it will show as below

Here is a demo .

pbix attached

In addition , you can refer this post if help.

https://community.powerbi.com/t5/Desktop/Can-I-have-a-tooltip-per-entry-to-a-table/td-p/928605

Best Regards,
Community Support Team _ Eason
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