Generate Discount Code for specific amount?

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I am a new eCommerce owner and using RoboNegotiator to settle final selling price with my customers.  In other words, I might list a product for $500 but my lowest selling price is $450 (10% lower than listed price  - which RoboNegotiator knows). At the end, RoboNegotiator finds me a buyer for $460 or $465 or $480 (they have AI based matching and negotiation engine). My question is how can I provide a unique discount-code to that buyer for that specific price (discount code needs to subtract either $50 or $35 or $25 or $20 from listed price).

Not being expert in how discounting, discount code works (at checkout time), I would love to hear feedback from my fellow eCommerce owners.