wont except negative value on create bill. intermittently

Hi

I was trying to input a supplier return by creating a bill with a negitive value. i followed the instructions on the myob help site. the following error came up

An unrecognised error occurred - PurchaseEvent_NotMatchLiabilityAccountForTrackingPayables An unrecognised error occurred - PurchaseEvent_NotLiabilityAccountWarning A problem occurred with one of the transaction lines that has a negative decimal amount. Please delete the line, add a new line, and try again.

I tried to enter this bill/return several times with no luck.

I find that sometime if i enter a negitive value it works and someitmes it dosnt. eg bill is 1 cent out and i want to make it equal right amount. wont always allow me to change it. if i make it a discount it will work

thanks