Setting up Advance Payment Term

Hi,

Please help on the following.

I was trying to set up advance payment term in SAP. As per the term, customer will be 15 days of average purchase amount as advance in the start of the month and continue to have that balance throughout. If he maintains such balance for entire month, he gets 2.5 % as cash discount on the purchase made while making payment. In case he fails to maintain 15 days of average sales value as advance , cash discount % will be  2%.

Please suggest whether such term is possible or not??

Thanking you in anticipation.