I have no experience of this so please do not bite.
Have any of you in this situation, ie being offered such a discount asked for a directors guarantee on the payment?
Any reputable company director who is not trying to prop up a failing cash flow should have no problem signing such a guarantee. this would negate his limited liability and then the only problem you would have is whether he has the assetts to back his word.
I may be totally wrong but someone should be able to clarify this. The thing is, if they wont committ to you, don't committ to them.