Edit Adjustment on Price History

Dear All
I Have discount program that we can give a dicount maximum $50, sales can give max $50 or less
I confuse how to set it. maybe i can made adjustment definition, details and schedule with Blank Amount
and after that form price history on Sales Order Details can I can Edit that Adjustment

But I can not Edit it... i just have option add another adjustment manualy

My question :
1. How can I edit Adjustment Amount on my Pricelist History on Sales Order Details
2. How can set maximum amount for add some adjustment

Hope somebody can advise me

Thanks

Juned