Capturing Advance monies at Sales order and delivery

Hi Experts,

Advance Monies needs to be collected as per the below guidelines

1.25% advance along with order(Can be captured
2.50%at the time of delivery
3.balance 25% after pgi and while giving the invoice

If not collected the activity should not be allowed to save or proceed

Note but there is an exception

If i want to relax it for a particular customer(there is no grouping for cust)then it should not go through the any of the above checks.

Regards,

Anese Khan