Direct GRN with missed prices entry

When I make a Direct GRN, and I have missed to enter prices (specifics of business), when I try to enter them later at the payments, I get the message about the allowed difference. I can change it, but I see 2 problems here:

1. Having price at 0 any change is beyond 100% (so I need to be able to turn the function on/off);
2. At some cases the limits between 0-100% are inadequate. For example, sometimes forced by circumstances man sells with loss, or sometimes a price can jump with more than 100% in some short time (for example inflation).