My letter wouldn't be threatening.
Maybe it would go something like this
It has come to our attention that your website www.somebusiness. has link out to our website www.thebestinitsfield.
We welcome such links from high quality and respected websites such as yours.
In our endeavour to provide the best resource in our field, we routinely screen the websites that use our resource as a reference, because Google has a policy( a link here to Google's blog about disavow links) of penalising sites that are seen to have low quality links to it.Google suspects those low quality links to be bought by the website that they are linking to and thus outside their guidelines to successful ranking.
A website such as ours attracts many new links daily and to protect our integrity, which is in your interest and ours, we have come up with a system where quality sites such as yours who link to us, pay a token price of $5 paypal to verify that your link to www.thebestinitsfield is valid.
In this way, we protect both the integrity of your website and ours.
Our paypal address is pay-or-disavow@thebestinitsfield. Please contact us if you have any enquiries.
What do you think?:)