What is the difference between 妥善 and 妥当?
Both '妥善' and '妥当' means  "appropriate" or "proper" . 

There are minor difference: 

'妥善' implies  "(satisfactorily) appropriate"

'妥当' implies "(correctly) appropriate"

Example:


對員工遣散有妥善安排 = There is a proper arrangement for severance. (everyone should be satisfied)
對員工遣散有妥当安排 = There is a proper arrangement for severance. (everything by the book without mistake)
妥善地佈防 = properly set up defensive positions (satisfactory on all accounts)
妥当地佈防 = properly set up defensive positions (follow all protocols correctly)


If someone said he will give you a 妥善的補償, he is trying to assure you that the compensation will be fair and satisfactory; If he said he will give you a 妥当的補償, it means you will get whatever you are entitled to. No more, no less.