Discount calculation erors for AP payments.

 Hi,
Our business is facing the following issue. We are on R12(12.0.6).

They have an invoice for the amount of $26,250,with the invoice date 02-APR-2010 and with the payment terms 1%10N30(1% discount, when paid with in 10 days and the payment is due with in 30 days from invoice date).
However when the payment is made on 15-APR-2010(well after 10days from the invoice date), the system is providing the discount of 1% and payment amount is coming up to -25,987.5.
Our business is worried that the vendor might not be happy that the they are underpaying them.
Is this a known issue, can some one provide any thoughts on this?  A quick response is very highly appreciated.