Offering clients discounts for backlinks?

I am considering creating a password-protected page on my subdomain to send out to clients at the same time we invoice them.

it offers a discount on their invoice in exchange for a link to our website. What do you guys think about this? Will it make our brand look shady? Can we get caught by Google? Is this a common practice? I read it as a link-building tactic from Joshua Hardwick from Ahrefs and it intrigued me.

Thanks.