Instead of a customer calling you one day, placing a critical order and then being told them they must pay a large balance from their open account before any more shipments can be made, be fair with the customer, call him well in advance and tell him he is approaching his credit limit. Let him know he has passed a substantial portion of his credit limit. Do not put his business in jeopardy by cutting him off and holding him hostage for money. You might trying to make a call when his balance hits 50%, tell him he has money left but you do not want him to reach his limit and be cutoff. Ask for updated financial statements from his private firm or talk to his bank (if given permission) to be reassured he has adequate financing and then see if you can raise his credit limit. Do all of these things well in advance. Have your collection personnel contact him politely now for money.