i want to create multiple purchse requisition(pr) to single sales order where is the control?

in third party process if low risk customers credit limit exceed,where can we block the customers? where is the control?

After creating sales order user changing the quantity in purchase order ? what happens ? and where is the control?

in third party process when vendor is delivering the goods to customers which invoice vendor will send along with dispatch and whoose name?    plesase give me solution.....