Firstly what is the data type that Power Query is using for the amount field you are referring to ?
If this is ABC123 or 123, you need to find the Changed Data Type step that made it this type and remove/change it.
You need the column to show a data type of 1.2 Decimal (or if 4 decimals is enough then $ Currency)
It is most likely that the very 2nd step was Changed Data Type step. If this is the case and then click on that step and check if the column at that point already 123 (or ABC123).
Click on the 123 (or ABC123) and select say 1.2 Decimal no.
If it then shows the option to "Replace current" the select that.
If it creates a new Changed Type step you will need to delete the original change type step.
Note: you can not change it "back to" 1.2 or currency after a step that has converted it to "123 whole number.
The initial conversion will have lost the decimals and changing it back after does not recover the lost data,
.