Return Leg from Hong Kong Need Advice

I am in Shenzhen with a Z Business return out of HKG on 4/29. The entire ticket cost my company about $7500. I am GS and need to return earlier. I know a business Z is an issue but when I go online to try and see what it would cost me to get out of here earlier I am only offered economy seats at a change and new ticket cost of nearly $6000. If I go to UAL and request a 1 way economy from HKG to SAN it will sell me a ticket for $1100. I am so confused. How can it cost More to exchange a business class return for an economy than to buy a 1 way economy? I know you're going to advise me to call the GS desk and I will as soon as my return date is Firm but wanted some expert input on this and wondered if anyone could shed light on this?

What if I cancelled the return would I get credit for the unused portion I could use toward another ticket?

Thank you for any help and advice.