Put this in SO_Sales Order Header, Column-PostValidate Customer No. PM me if you need a hand adding it.
retVal = 0
custBal = 0
current = 0
age1 = 0
age2 = 0
age3 = 0
age4 = 0
terms = """"
SET oCust = oBusObj.AsObject(oBusObj.GetChildHandle(""CustomerNo""))
retVal = oCust.GetValue(""CurrentBalance"", current)
retVal = oCust.GetValue(""AgingCategory1"", age1)
retVal = oCust.GetValue(""AgingCategory2"", age2)
retVal = oCust.GetValue(""AgingCategory3"", age3)
retVal = oCust.GetValue(""AgingCategory4"", age4)
retVal = oCust.GetValue(""TermsCode$"", terms)
custBal = current + age1 + age2 + age3 + age4
if custBal < 0 and terms = ""07"" then
retVal = oScript.InvokeButton(""BT_Credit"")
end if