Payables Management

Hello-

my client is in a position where the AP to Inventory Ratio is approximately 50%; meaning the company is not properly leveraging payables. It is believed that most of this is due to poorly negotiated payment terms. As the SAP system pays vendors according to the terms that have been negotiated does anyone know of a way to get the system to pay on a 'terms plus' basis for certain vendors? For example, if my payment terms are n/20 but the inventory turns every 35 days on average, is there a way to tell the system to pay a particular vendor at term + 15?