I would like to know how can I calculate the SUM of the field that contains value where the unit is tones and the SUM of the value where the unit is Ha.

You can create a couple of calculated fields to normalize the data. Sample Let's create a table or view and add a new column like below

Now let's create a calculated field for the first unit in tones

Second calculated field with the unit ha

In the final view, you can combine all those and create a tabular view

 This will give you the total amount corresponding to the units they have.