The percentage of guarantee, up to the maximum allowed by this section, is a matter for negotiation between the lender and the Agency.
    
    
        
        (a) 
         The maximum guarantee is 90 percent of eligible loss.
     
    
        
        (b) 
         The lender will retain a minimum of 5 percent of the total loan amount. The retained amount must be from the unguaranteed portion of the loan and cannot be participated to another lender.