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Formosul is the mildest of the dye discharge chemicals that you can choose. All the other dye discharge agents require soda ash and a higher pH, so they are all harsher. If you try discharge at all, using Formosul at an acid pH would be the least damaging choice. Look at the three recipes on ProChem's page on how to use Formosul.

There is another option which does not call for any discharge chemical at all. It is much gentler, and it might work, if it happens that the dye used on the cashmere was a leveling acid dye, which is very common. (It's the type of acid dye used in all-purpose dyes such as Rit, as well as better-regarded dyes such as Landscapes acid dye and Cushing acid dye.) What you do is heat the cashmere in a pot of water with Glauber's salt, sodium sulfate. It will not remove all the dye, but may lighten it by about 25%. I posted this before here on the forum:

Linda Knutson says to weigh the material, while it is dry, then use one-fifth this weight of Glauber's salt, simmering with it for 30 to 60 minutes; the amount of water required would typically be 25 times the weight of the fabric, as in normal immersion dyeing, so for each pound of wool/nylon fabric (454 grams) you'd want three gallons of water, or 11 liters.

-Paula